enhanced ai for stairs, obstacles, windows

was #7240. now with better notes. as a single commit after i got out of sync here.

monsters arrive at more than one stair
monsters push monsters off of stairs and sometimes in combat too
monsters push the player off of the stairs better 
monsters path out of windows
monsters path around obstacles.
judoka resist pushing and can throw monster trying it.

mind you, they are zombies though.  so this was solved not by introducing intelligence but by a little more dumbness to effectively solve these longstanding issues.