user: disable setgroups before writing the gid_map
user: disable setgroups before writing the gid_map

Newer linux versions (3.18+) forbid writing to the gid_map if the setgroups
syscall is enabled. This also removes the initgroups() call, since it uses
setgroups internally.

Thanks to Christian Neukirchen for the original patch.

ghedo/pflask,jwessel/pflask,jwessel/pflask,gdm85/pflask,ghedo/pflask,gdm85/pflask