Adding system for retributive monster strikes.

Ported and buffed the current scheme for damage on hit from shock zombies and shadows to a new framework for triggering things that happen when something is hit.  The longer-term idea is to make combat a bit more dynamic and strategic by incentivizing/penalizing different types of attacks beyond basic damage type resistance/weakness and armour piercing.  This will also hopefully help keep the monster-specific stuff out of the generic attack/damage routines.

This is roughly analogous to the current sp_attack system.

As for the zapback buff, this is intended to make the penalty from striking a shock zombie worth noticing.  a 50% chance of 1 damage that can be blocked by wearing gloves is pretty weaksauce.

Looking for feedback, as this might also belong under on_gethit, but it doesn't look like that's been implemented for monsters yet.  I'd be happy to add a few more things to build on (quill attacks, spores, disarming, etc.) but figured some input would be good before ploughing forward.