otherwise this BADLY breaks if -funsigned-char is passed to gcc

fcntl values 1024 and up are universal, arch-independent. later I'll
add some of the other linux-specific ones for notify, leases, pipe
size, etc. here too.

F_* is in the reserved namespace so no feature test is needed

DECIMAL_DIG is not the same as LDBL_DIG

type_DIG is the maximimum number of decimal digits that can survive a
round trip from decimal to type and back to decimal.

DECIMAL_DIG is the minimum number of decimal digits required in order
for any floating point type to survive the round trip to decimal and
back, and it is generally larger than LDBL_DIG. since the exact
formula is non-trivial, and defining it larger than necessary may be
legal but wasteful, just define the right value in bits/float.h.

the old abi was intended to duplicate glibc's abi at the expense of
being ugly and slow, but it turns out glib was not even using that abi
except on non-gcc-compatible compilers (which it doesn't even support)
and was instead using an exceptions-in-c/unwind-based approach whose
abi we could not duplicate anyway without nasty dwarf2/unwind
integration.

the new abi is copied from a very old glibc abi, which seems to still
be supported/present in current glibc. it avoids all unwinding,
whether by sjlj or exceptions, and merely maintains a linked list of
cleanup functions to be called from the context of pthread_exit. i've
made some care to ensure that longjmp out of a cleanup function should
work, even though it is not required to.

this change breaks abi compatibility with programs which were using
pthread cancellation, which is unfortunate, but that's why i'm making
the change now rather than later. considering that most pthread
features have not been usable until recently anyway, i don't see it as
a major issue at this point.

it's a keyword in c++ (wtf). i'm not sure this is the cleanest
solution; it might be better to avoid ever defining __NEED_wchar_t on
c++. but in any case, this works for now.

the arm syscall abi requires 64-bit arguments to be aligned on an even
register boundary. these new macros facilitate meeting the abi
requirement without imposing significant ugliness on the code.

at the same time, make struct statfs match the traditional definition
and make it more useful, especially the fsid_t stuff.

really wchar_t should never vary, but the ARM EABI defines it as an
unsigned 32-bit int instead of a signed one, and gcc follows this
nonsense. thus, to give a conformant environment, we have to follow
(otherwise L""[0] and L'\0' would be 0U rather than 0, but the
application would be unaware due to a mismatched definition for
WCHAR_MIN and WCHAR_MAX, and Bad Things could happen with respect to
signed/unsigned comparisons, promotions, etc.).

fortunately no rules are imposed by the C standard on the relationship
between wchar_t and wint_t, and WEOF has type wint_t, so we can still
make wint_t always-signed and use -1 for WEOF.

this behavior (opening fds 0-2 for a suid program) is explicitly
allowed (but not required) by POSIX to protect badly-written suid
programs from clobbering files they later open.

this commit does add some cost in startup code, but the availability
of auxv and the security flag will be useful elsewhere in the future.
in particular auxv is needed for static-linked vdso support, which is
still waiting to be committed (sorry nik!)

if gcc decided to move this across a conditional that checks validity
of the thread register, an invalid thread-register-based read could be
performed and raise sigsegv.

some program was undefining AF_NETLINK and thereby breaking AF_ROUTE...

1. search was wrongly beginning with lib itself rather than dso head
2. inconsistent resolution of function pointers for functions in plt

first, use $LD_LIBRARY_PATH unless suid. if that fails, read path from
/etc/ld-musl-$ARCH.path and fallback to a builtin default.

some notes:
- library search path is hard coded
- x86_64 code is untested and may not work
- dlopen/dlsym is not yet implemented
- relocations in read-only memory won't work

this slightly cuts down on the degree musl "fights with" gcc, but more
importantly, it fixes a critical bug when gcc inlines a variadic
function and optimizes out the variadic arguments due to noticing that
they were "not used" (by __builtin_va_arg).

we leave the old code in place if __GNUC__ >= 3 is false; it seems
like it might be necessary at least for tinycc support and perhaps if
anyone ever gets around to fixing gcc 2.95.3 enough to make it work..

strictly speaking this and a few other ops should be factored into
asm.h or the file should just be renamed to asm.h, but whatever. clean
it up someday.

this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.

the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.

these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.

x86_64 asm is untested and may need a second try to get it right.

some of these definitions were just plain wrong, others based on
outdated ancient "non-64" versions of the kernel interface.

as much as possible has now been moved out of bits/*

these changes break abi (the old abi for these functions was wrong),
but since they were not working anyway it can hardly matter.

trash in the upper 32 bits was making the kernel sleep forever in
select on 64-bit systems.

the basic idea is that the only things in alltypes.h should be types
that either vary from system to system (in practice, not just in
theoretical la-la land - this is the implementation so we choose what
constraints we want to impose on ports) or which are needed by
multiple system headers.

actually FLT_ROUNDS needs to expand to a static inline function that
obtains the current rounding mode and returns it, but that will be
added later with fenv.h stuff.

POSIX clearly specifies the type of msg_iovlen and msg_controllen, and
Linux ignores it and makes them both size_t instead. to work around
this we add padding (instead of just using the wrong types like glibc
does), but we also need to patch-up the struct before passing it to
the kernel in case the caller did not zero-fill it.

if i could trust the kernel to just ignore the upper 32 bits, this
would not be necessary, but i don't think it will ignore them...