maint: split giant posix.c file into manageable chunks.

* ext/posix/posix.c: Separate into sections...
* ext/posix/_helpers.c, ext/posix/_helpers.h, ext/posix/ctype.c,
ext/posix/dirent.c, ext/posix/errno.c, ext/posix/fcntl.c,
ext/posix/fnmatch.c, ext/posix/getopt.c, ext/posix/glob.c,
ext/posix/grp.c, ext/posix/libgen.c, ext/posix/poll.c,
ext/posix/pwd.c, ext/posix/sched.c, ext/posix/signal.c,
ext/posix/stdio.c, ext/posix/stdlib.c, ext/posix/syslog.c,
ext/posix/termio.c, ext/posix/time.c, ext/posix/unistd.c,
ext/posix/utime.c: ...here.
* ext/posix/posix.c: #include sources back into here.
* build-aux/config.ld.in (file): Adjust accordingly.
* build-aux/sanity-cfg.mk: Likewise.
* lib/posix.lua: Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>