posix: use correct POSIX gethostid API.

Closes #153.
* specs/posix_spec.yaml (hostid): Move from here...
* specs/posix_compat.yaml (hostid): ...to here.
* specs/posix_unistd_spect.yaml (gethostid): Specify with correct
POSIX API name.
* ext/posix/unistd.c (Phostid): Rename from this...
(Pgethostid): ...to this.
* lib/posix/compat.lua (hostid): An alias to gethostid for
backwards compatibility.
* NEWS: Update.

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