core: create/remove unit bus name slots always together

core: create/remove unit bus name slots always together

When a service unit watches a bus name (i.e. because of BusName= being
set), then we do two things: we install a match slot to watch how its
ownership changes, and we inquire about the current owner. Make sure we
always do both together or neither.

This in particular fixes a corner-case memleak when destroying bus
connections, since we never freed the GetNameOwner() bus slots when
destroying a bus when they were still ongoing.