core: rework the "no_gc" unit flag to become a more generic "perpetual" flag

So far "no_gc" was set on -.slice and init.scope, to units that are always
running, cannot be stopped and never exist in an "inactive" state. Since these
units are the only users of this flag, let's remodel it and rename it
"perpetual" and let's derive more funcitonality off it. Specifically, refuse
enqueing stop jobs for these units, and report that they are "unstoppable" in
the CanStop bus property.