Rolling boulder traps and pacifism

If you stepped on an unknown rolling boulder trap, and that rolling boulder
hit a monster and killed it, you would be called a killer.  This makes
playing a pacifism conduct game rather difficult.
- track boulders from unknown rolling boulder traps, and don't charge/credit
hero if they kill monsters. This is done by temporarily setting otrapped on
such boulders.
- boulders from known traps are still charged/credited to the hero
- fix a couple places in ohitmon where is_poisonable wasn't checked along
with opoisoned.

include/obj.h

@@ -70,6 +70,7 @@ struct obj {
 	Bitfield(olocked,1);	/* object is locked */
 	Bitfield(obroken,1);	/* lock has been broken */
 	Bitfield(otrapped,1);	/* container is trapped */
+				/* or accidental tripped rolling boulder trap */
 #define opoisoned otrapped	/* object (weapon) is coated with poison */
 
 	Bitfield(recharged,3);	/* number of times it's been recharged */