Improve DAEMON_SHUTDOWN for partitionable slots

A partitionable slot always appears idle, so it would terminate
after GLIDEIN_Max_idle after running, even if all of its subslots
were occupied.  This should arrange so it only terminates if the
subslots have been reclaimed; this isn't perfect, but I'm not sure
we can do more without additional information in the machine ClassAd.
Merge branch 'branch_v2plus_3399' into branch_v2plus