(#12459) drop supplementary groups when permanently dropping UID

On certain platforms, Process::UID.change_privilege doesn't handle
changing supplementary groups. In that case, we need to always
explicitly initgroups, regardless of whether we're using
change_privilege, or change_group/change_user.

Analysis by Dominic Cleal <dcleal@redhat.com>

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>