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.

fundamentally there is no good reason these functions need to set an
orientation (morally it should be possible to write a wchar_t[] memory
stream using byte functions, or a char[] memory stream using wide
functions), but it's a part of the specification that they do. aside
from being able to inspect the orientation with fwide, failure to set
the orientation in open_wmemstream is observable if the locale changes
between open_wmemstream and the first operation on the stream; this is
because the encoding rule (locale) for the stream is required to be
bound at the time the stream becomes wide-oriented.

for open_wmemstream, call fwide to avoid duplicating the logic for
binding the encoding rule. for open_memstream it suffices just to set
the mode field in the FILE struct.

bring these functions up to date with the current idioms we use/prefer
in fmemopen and fopencookie.

the specification for these functions requires that the buffer/size
exposed to the caller be valid after any successful call to fflush or
fclose on the stream. the implementation's approach is to update them
only at flush time, but that misses the case where fflush or fclose is
called without any writes having taken place, in which case the write
flushing callback will not be called.

to fix both the observable bug and the desired invariant, setup empty
buffers at open time and fail the open operation if no memory is
available.

functions which open in-memory FILE stream variants all shared a tail
with __fdopen, adding the FILE structure to stdio's open file list.
replacing this common tail with a function call reduces code size and
duplication of logic. the list is also partially encapsulated now.

function signatures were chosen to facilitate tail call optimization
and reduce the need for additional accessor functions.

with these changes, static linked programs that do not use stdio no
longer have an open file list at all.

per interpretation for austin group issue #626, fflush(0) and exit()
must block waiting for a lock if another thread has locked a memory
stream with flockfile. this adds some otherwise-unnecessary
synchronization cost to use of memory streams, but there was already a
synchronization cost calling malloc anyway.

previously the stream was only added to the open file list in
single-threaded programs, so that upon subsequent call to
pthread_create, locking could be turned on for the stream.

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.

now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.

the addition is safe and cannot overflow because both operands are
positive when considered as signed quantities.

the expression -off is not safe in case off is the most-negative
value. instead apply - to base which is known to be non-negative and
bounded within sanity.