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.

the value 0x7f00 (as if by _exit(127)) is specified only for the case
where the child is created but then fails to exec the shell, since
traditional fork+exec implementations do not admit reporting an error
via errno in this case without additional machinery. it's unclear
whether an implementation not subject to this failure mode needs to
emulate it; one could read the standard as requiring that. if so,
additional code will need to be added to map posix_spawn errors into
the form system is expected to return. but for now, returning -1 to
indicate an error is significantly better behavior than always
reporting failures as if the shell failed to exec after fork.

patch by Jens Gustedt.
previously, the intended policy was to use __environ in code that must
conform to the ISO C namespace requirements, and environ elsewhere.
this policy was not followed in practice anyway, making things
confusing. on top of that, Jens reported that certain combinations of
link-time optimization options were breaking with the inconsistent
references; this seems to be a compiler or linker bug, but having it
go away is a nice side effect of the changes made here.

this avoids duplicating the fragile logic for executing an external
program without fork.

ideally, system would also be cancellable while running the external
command, but I cannot find any way to make that work without either
leaking zombie processes or introducing behavior that is far outside
what the standard specifies. glibc handles cancellation by killing the
child process with SIGKILL, but this could be unsafe in that it could
leave the data being manipulated by the command in an inconsistent
state.

__release_ptc() is only valid in the parent; if it's performed in the
child, the lock will be unlocked early then double-unlocked later,
corrupting the lock state.

since we target systems without overcommit, special care should be
taken that system() and popen(), like posix_spawn(), do not fail in
processes whose commit charges are too high to allow ordinary forking.
this in turn requires special precautions to ensure that the parent
process's signal handlers do not end up running in the shared-memory
child, where they could corrupt the state of the parent process.
popen has also been updated to use pipe2, so it does not have a
fd-leak race in multi-threaded programs. since pipe2 is missing on
older kernels, (non-atomic) emulation has been added.
some silly bugs in the old code should be gone too.