Move UserLookup directly into sysinit, since that's the only place it is used (and the only bit of utils that was used in sysinit)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update sysinit to also grab the supplementary groups of the given user from /etc/group

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)