Always use primary group for initgroups

[mattmix]

When users are not members of their primary group[1], initgroups(3) being called with the group listed in the group_list parameter causes the primary group to be missing in supplemental group list for the launched job.

This change will cause init_groups to always call initgroups(3) with the user's primary group so that it will continue to exist later on when setgid is called with the group from the group_list.

Redacted example:

[user@login ~] # groups
primarygrp secondarygrp

# Unpatched node:
[user@login ~] # qsub -I -W group_list=secondarygrp
[user@compute ~] groups
secondarygrp

# Patched node:
[user@login ~] # qsub -I -W group_list=secondarygrp
[user@compute ~] groups
secondarygrp primarygrp

[1] Happens when the user's entry in LDAP assigns them to a group, but they aren't added to the group member list.