fix(make_chroot): Simplify chroot user creation.

The commands useradd/usermod will silently skip adding users to
secondary groups that are not in /etc/group. The idea being that the
tools should not create groups that conflict with existing LDAP/NIS
groups but why trying to do so isn't a fatal error I don't know.

Overall the code is rather complicated and tries to modify instead of
add when possible to allow running the SDK as the 'core' user. To keep
things simple gut this code, make the 'core' user special, and add
secondary groups via the 'gpasswd' command so that errors are reported
instead of silently ignored.

One functional change: the default groups have changed to kvm and
portage. The old list excluded kvm and included lots of extra cruft.