manager: reduce complexity of unit_gc_sweep (#3507)

manager: reduce complexity of unit_gc_sweep (#3507)

When unit is marked as UNSURE, we are trying to find if it state was
changed over and over again. So lets not go through the UNSURE states
again. Also when we find a GOOD unit lets propagate the GOOD state to
all units that this unit reference.

This is a problem on machines with a lot of initscripts with different
starting priority, since those units will reference each other and the
original algorithm might get to n! complexity.

Thanks HATAYAMA Daisuke for the expand_good_state code.