Accessibility: more message locations

Add a new pline_mon() which sets the message location
to the monster location.

Add locations to several trap messages.

src/steal.c

@@ -846,7 +846,7 @@ mdrop_obj(
     }
     /* obj_no_longer_held(obj); -- done by place_object */
     if (verbosely && cansee(omx, omy))
-        pline_xy(mon->mx, mon->my, "%s drops %s.", Monnam(mon), obj_name);
+        pline_mon(mon, "%s drops %s.", Monnam(mon), obj_name);
     if (!flooreffects(obj, omx, omy, "fall")) {
         place_object(obj, omx, omy);
         stackobj(obj);