doc: document returned error numbers in many APIs.

* ext/posix/fcntl.c, ext/posix/poll.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/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,
lib/posix/compat.lua, lib/posix/deprecated.lua: Document third
returned error number for many APIs.

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