posix: split posix subfiles into separate modules.

* ext/posix/_helpers.h: Removed.  Content moved from here...
* ext/posix/_helpers.c: ...to here. Adjust all callers.
* ext/posix/posix.c (R, luaopen_posix_c): Move registered
functions and constants from here...
* 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/sys/msg.c, ext/posix/sys/resource.c,
ext/posix/sys/socket.c, ext/posix/sys/stat.c,
ext/posix/sys/statvfs.c, ext/posix/sys/time.c,
ext/posix/sys/times.c, ext/posix/sys/utsname.c,
ext/posix/sys/wait.c, ext/posix/syslog.c, ext/posix/termio.c,
ext/posix/time.c, ext/posix/unistd.c, ext/posix/utime.c: ...to
here. Provide individual `luaopen_<submodule>` entry functions.
build-aux/config.ld.in, build-aux/sanity-cfg.mk: Adjust
accordingly.
* ext/posix/posix.c: Reduce to a single compilation unit that
includes the sources for all the other submodules.
* lib/posix.lua (openpty): Move from here...
* lib/posix/util.lua (openpty): New file. ...to here.
* lib/posix/sys.lua: Move definitions from here...
* lib/posix.lua: ...to here.
* lib/posix/sys.lua: Re-export backwards compatibility APIs from
here too.
* lib/posix.lua: Re-export all submodule entry points from here
too to retain the flat namespace of old releases.
* local.mk: Adjust accordingly.
* NEWS: Update.

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