the previous few commits ended up leaving the thread count and signal
mask wrong for atexit handlers and stdio cleanup.

now that blocking signals prevents any application code from running
while the last thread is exiting, the cas logic is no longer needed to
prevent decrementing below zero.

the thread count (1+libc.threads_minus_1) must always be greater than
or equal to the number of threads which could have application code
running, even in an async-signal-safe sense. there is at least one
dangerous race condition if this invariant fails to hold: dlopen could
allocate too little TLS for existing threads, and a signal handler
running in the exiting thread could claim the allocated TLS for itself
(via __tls_get_addr), leaving too little for the other threads it was
allocated for and thereby causing out-of-bounds access.

there may be other situations where it's dangerous for the thread
count to be too low, particularly in the case where only one thread
should be left, in which case locking may be omitted. however, all
such code paths seem to arise from undefined behavior, since
async-signal-unsafe functions are not permitted to be called from a
signal handler that interrupts pthread_exit (which is itself
async-signal-unsafe).

this change may also simplify logic in __synccall and improve the
chances of making __synccall async-signal-safe.

for the duration of the vm-sharing clone used by posix_spawn, all
signals are blocked in the parent process, including
implementation-internal signals. since __synccall cannot do anything
until successfully signaling all threads, the fact that signals are
blocked automatically yields the necessary safety.

aside from debloating and general simplification, part of the
motivation for removing the explicit lock is to simplify the
synchronization logic of __synccall in hopes that it can be made
async-signal-safe, which is needed to make setuid and setgid, which
depend on __synccall, conform to the standard. whether this will be
possible remains to be seen.

C++11, the first C++ with stdint.h, requires the previously protected
macros to be exposed unconditionally by stdint.h. apparently these
checks were an early attempt by the C committee to guess what the C++
committee would want, and they guessed wrong.

this caused sigsetjmp not to save the signal mask but instead to
clobber it with whatever happened to be in the sigjmb_buf prior to the
call.

this allows /etc/ld-musl-$(ARCH).path to contain one path per line,
which is much more convenient for users than the :-delimited format,
which was a source of repeated and unnecessary confusion. for
simplicity, \n is also accepted in environment variables, though it
should probably not be used there.

at the same time, issues with overly long paths invoking UB or getting
truncated have been fixed. such issues should not have arisen with the
environment (which is size-limited) but could have been generated by a
path file larger than 2**31 bytes in length.

the getifaddrs interface seems to have been invented by glibc, and
they expose socket.h, so for us not to do so is just gratuitous
incompatibility with the interface we're mimicing.

the standard is clear that the old behavior is conforming: "In this
case, [EILSEQ] shall be stored in errno and the conversion state is
undefined."

however, the specification of mbrtowc has one peculiarity when the
source argument is a null pointer: in this case, it's required to
behave as mbrtowc(NULL, "", 1, ps). no motivation is provided for this
requirement, but the natural one that comes to mind is that the intent
is to reset the mbstate_t object. for stateful encodings, such
behavior is actually specified: "If the corresponding wide character
is the null wide character, the resulting state described shall be the
initial conversion state." but in the case of UTF-8 where the
mbstate_t object contains a partially-decoded character rather than a
shift state, a subsequent '\0' byte indicates that the previous
partial character is incomplete and thus an illegal sequence.

naturally, applications using their own mbstate_t object should clear
it themselves after an error, but the standard presently provides no
way to clear the builtin mbstate_t object used when the ps argument is
a null pointer. I suspect this issue may be addressed in the future by
specifying that a null source argument resets the state, as this seems
to have been the intent all along.

for what it's worth, this change also slightly reduces code size.

the interface contract for mbtowc admits a much faster implementation
than mbrtowc can achieve; wrapping mbrtowc with an extra call frame
only made the situation worse.

since the regex implementation uses mbtowc already, this change should
improve regex performance too. it may be possible to improve
performance in other places internally by switching from mbrtowc to
mbtowc.

this simple change, in my measurements, makes about a 7% performance
improvement. at first glance this change would seem like a
compiler-specific hack, since the modified code is not even used.
however, I suspect the reason is that I'm eliminating a second path
into the main body of the code, allowing the compiler more flexibility
to optimize the normal (hot) path into the main body. so even if it
weren't for the measurable (and quite notable) difference in
performance, I think the change makes sense.

SA and SB are used as the lowest and highest valid starter bytes, but
the value of SB was one-past the last valid starter. this caused
access past the end of the state table when the illegal byte '\xf5'
was encountered in a starter position. the error did not show up in
full-character decoding tests, since the bogus state read from just
past the table was unlikely to admit any continuation bytes as valid,
but would have shown up had we tested feeding '\xf5' to the
byte-at-a-time decoding in mbrtowc: it would cause the funtion to
wrongly return -2 rather than -1.

I may eventually go back and remove all references to SA and SB,
replacing them with the values; this would make the code more
transparent, I think. the original motivation for using macros was to
allow misguided users of the code to redefine them for the purpose of
enlarging the set of accepted sequences past the end of Unicode...

also include fallback code for broken kernels that don't support the
flags. as usual, the fallback has a race condition that can leak file
descriptors.

this is a bit ugly, and the motivation for supporting it is
questionable. however the main factors were:
1. it will be useful to have this for certain internal purposes
anyway -- things like syslog.
2. applications can just save argv[0] in main, but it's hard to fix
non-portable library code that's depending on being able to get the
invocation name without the main application's help.

GNU used several extensions that were incompatible with C99 and POSIX,
so they used alternate names for the standard functions.

The result is that we need these to run standards-conformant programs
that were linked with glibc.

supports ipv4 and ipv6, but not the "extended" usage where
usage statistics and other info are assigned to ifa_data members
of duplicate entries with AF_PACKET family.

the preprocessor can reliably determine the signedness of wchar_t.
L'\0' is used for 0 in the expressions so that, if the underlying type
of wchar_t is long rather than int, the promoted type of the
expression will match the type of wchar_t.

since shadow does not yet support enumeration (getspent), the
corresponding FILE-based get and put versions are also subbed out for
now. this is partly out of laziness and partly because it's not clear
how they should work in the presence of TCB shadow files. the stubs
should make it possible to compile some software that expects them to
exist, but such software still may not work properly.

remove redundant headers and comments; this file is completely trivial
now. also, avoid temp var.