posix: report correct bad option number.

* lposix.c (badoption): Report the passed option number as being
the bad option.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
posix: mark unused parameters for gcc.

* lposix.c (UNUSED): New macro, expands to __unused__ with GNU C.
(FgetID, Fsysconf, Pcloselog, Psync): Use it to mark unused
function parameters.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
posix: remove unused variables.

* lposix.c (Precvfrom): Remove unused host and serv variables.
(Paccept): Likewise, and remove unused r as well.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
posix: remember to register Pshutdown function.

* lposix.c (R): Add Pshutdown MENTRY.

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