When we're given a numeric --user value, default to GID 0 if the numeric
ID doesn't correspond to a user entry in /etc/passwd that can provide us
with the user's primary group ID.
Make sure that GetAdditionalGroupsForUser() returns wrapped errors.
Also test various user:group forms.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>
Closes: #728
Approved by: mheon