cgfsng: don't require that systemd subsystem be mounted

cgfsng: don't require that systemd subsystem be mounted

Note that is_crucial_subsystem still lists name=systemd.  That is
used in cgfs and cgmanager.  Cgmanager is typically setup to create
name=systemd, so it is ok.  cgfs uses is_crucial_subsystem() only
to decide whether failure to create or chown a directory should be
terminal.  That's ok, because (a) if name=systemd is not mounted then
we won't hit that, and (b) if name=systemd is mounted, then we'd
really still like to set it up for containers.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>