fix failing acceptance test by fixing ps opts

(cherry picked from commit 175a3c61a0302602a5a3521ec567e2ec34181ab2)

Conflicts:
	libpromises/systype.c
Work around AIX forcing the primary group to be part of secondaries.

Fixes test case 17_users/unsafe/promise_outcomes.cf.

(cherry picked from commit c8b7a90bc7440c8f67eb4d2478d061de15c5ba77)