Scott (S3): Discard combos if target dies

Now, if a unit is killed, Scott will discard the tactic associated with
the downed unit on his next turn.  This is mainly to set a new primary
target if the current one falls.
Fix potential infinite loop in AI

AIContext now queues default move if moves are queued by the AI but none
are usable (as due to a depleted MP pool), instead of calling the AI
again.