doc: add LDocs for submodule constants.

* ext/posix/errno.c, ext/posix/fcntl.c, ext/posix/fnmatch.c,
ext/posix/sched.c, ext/posix/signal.c, ext/posix/stdio.c,
ext/posix/sys/msg.c, ext/posix/sys/socket.c, ext/posix/sys/wait.c,
ext/posix/syslog.c, ext/posix/unistd.c: Add LDocs for constants.
* NEWS: Update.

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