wand/spell/breath killer reason

Extend "killed by the touch of death inflicted by <monster>" to buzz().
"Killed by a bolt of cold" becomes "killed by a bolt of cold zapped by
<monster>" or "killed by a blast of cold" becomes "killed by a blast
of cold exhaled by <monster>" and so forth.

More work than expected; the zap code isn't passed enough context.

BZ_M_WAND() was producing the wrong value for wands zapped by monsters.

include/decl.h

@@ -793,6 +793,9 @@ struct instance_globals_b {
     /* pickup.c */
     boolean bucx_filter;
 
+    /* zap.c */
+    struct monst *buzzer; /* zapper/caster/breather who initiates buzz() */
+
     boolean havestate;
     unsigned long magic; /* validate that structure layout is preserved */
 };