Implement heal tagging

As with damage, healing can now also be tagged, to allow statuses and
AIs to react differently to different sources of HP recovery.  For
example, a unit in Skeleton can now be ended with a Tonic, but won't be
affected in the least by ReGen healing.
RSB II: Overhaul Zombie exploitation strategy

This now works on an "alert level" system, with the alert level rising
each time Zombie is exploited to do tons of damage to Robert with
healing items.  Note that the alert level also rises on its own as phase
transitions happen--exploitation in P4 will draw an immediate Omni, for
example.