--complete-if-new-ver: check for slot change
--complete-graph-if-new-ver: test slot change
_slot_operator_update_probe: handle slot conflict

The "insignificant change" detection from commit
0240b8b754ab5f642c50c863d4bec1dc3636ba29 is only valid when we are
not trying to resolve a slot conflict (see bug #457142).
slot_operator_replace_installed: check available

This fixes a case where it could try to pull in an unavailable SLOT.
repoman: warn if not FEATURES=sign, bug #457034