unistd: implement getpid and similar APIs.

Closes #152.
* specs/posix_unistd_spec.yaml: New file. Specify behaviours for
getegid, geteuid, getgid, getpgrp, getpid, getppid, getuid.
* specs/specs.mk (specs_SPECS): Add specs/posix_unistd_spec.yaml.
* ext/posix/unistd.c (getpid): Delete.
(setegid, geteuid, getgid, getpgrp, getpid, getppid, getuid): New
functions. Satisfy specifications.
* lib/posix.lua.in (getpid): Reimplement legacy API over new
POSIX bindings.
* NEWS: Update.

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