getpgrp: fix fallback handling

getpgrp: fix fallback handling

The test for generating a stub for getpgrp was wrong and would
result in duplicate symbols when building without __NR_getpgrp,
but with __NR_getpgid and __NR_getpid.

A closer look at the getpgrp implementation using getpgid showed
that getpid was being called to pass the current pid to getpgid.
This is unnecessary because passing 0 to getpgid returns the
pgid of the current process.

This patch cleans up the getpgrp implementation and the stub test.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>