* src/chroot.c (set_additional_groups): Add comments.
Given an empty or all-comma group list, diagnose it and return nonzero.
When more than one group is invalid, diagnose all of them,
not just the first.
(main): Honor --groups= also when --userspec= is not specified.
Now that set_additional_groups consistently diagnoses its failures,
don't diagnose it separately here.
* tests/chroot/credentials: Do not invoke with an empty group list.
* src/chroot.c (main): Initialize both "uid" and "gid". To -1.
This also allows one to set the user-ID or primary group-ID to 0,
in case it's not that already.
* tests/chroot/credentials: Test for the above.
