libc.h was intended to be a header for access to global libc state and
related interfaces, but ended up included all over the place because
it was the way to get the weak_alias macro. most of the inclusions
removed here are places where weak_alias was needed. a few were
recently introduced for hidden. some go all the way back to when
libc.h defined CANCELPT_BEGIN and _END, and all (wrongly implemented)
cancellation points had to include it.

remaining spurious users are mostly callers of the LOCK/UNLOCK macros
and files that use the LFS64 macro to define the awful *64 aliases.

in a few places, new inclusion of libc.h is added because several
internal headers no longer implicitly include libc.h.

declarations for __lockfile and __unlockfile are moved from libc.h to
stdio_impl.h so that the latter does not need libc.h. putting them in
libc.h made no sense at all, since the macros in stdio_impl.h are
needed to use them correctly anyway.

this is a helper function from strftime that's also used by wcsftime.

Commit 8a6bd730 added support for
padding specifier extensions to strftime, but did not modify wcsftime.
In the process, it added a parameter to __strftime_fmt_1 in strftime.c,
but failed to update the prototype in wcsftime.c. This was found by
compiling musl with LTO:

    src/time/wcsftime.c:7:13: warning: type of '__strftime_fmt_1' does \
        not match original declaration [-Wlto-type-mismatch]

Fix the prototype of __strftime_fmt_1 in wcsftime.c, and generate the
'pad' argument the same way as it is done in strftime.

this change is presently non-functional since the callees do not yet
use their locale argument for anything.

these functions were spuriously failing in the case where the buffer
size was exactly the number of bytes/characters to be written,
including null termination. since these functions do not have defined
error conditions other than buffer size, a reasonable application may
fail to check the return value when the format string and buffer size
are known to be valid; such an application could then attempt to use a
non-terminated buffer.

in addition to fixing the bug, I have changed the error handling
behavior so that these functions always null-terminate the output
except in the case where the buffer size is zero, and so that they
always write as many characters as possible before failing, rather
than dropping whole fields that do not fit. this actually simplifies
the logic somewhat anyway.

at present, since POSIX requires %F to behave as %+4Y-%m-%d and ISO C
requires %F to behave as %Y-%m-%d, the default behavior for %Y has
been changed to match %+4Y. this seems to be the only way to conform
to the requirements of both standards, and it does not affect years
prior to the year 10000. depending on the outcome of interpretations
from the standards bodies, this may be adjusted at some point.

this is a nonstandard extension.

LC_GLOBAL_LOCALE refers to the global locale, controlled by setlocale,
not the thread-local locale in effect which these functions should be
using. neither LC_GLOBAL_LOCALE nor 0 has an argument to the *_l
functions has behavior defined by the standard, but 0 is a more
logical choice for requesting the callee to lookup the current locale.
in the future I may move the current locale lookup the the caller (the
non-_l-suffixed wrapper).

at this point, all of the locale logic is dummied out, so no harm was
done, but it should at least avoid misleading usage.

unlike the strftime commit, this one is purely an ABI compatibility
issue. the previous version of the code would have worked just as well
with LC_TIME once LC_TIME support is added.

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.