fix more your corpse cases

- new cxname() to simplify doing the right thing in increasingly common cases
- use for bullwhip snagging
- in shopkeeper offer code
- in a couple other existing places rather than duplicating CORPSE checks
- use singular(...) in "swings" cases, since only one can hit.  Singular uses
  corpse_xname automatically when appropriate

src/mhitm.c

@@ -1188,7 +1188,7 @@ register struct obj *otemp;
 	Strcpy(buf, mon_nam(mdef));
 	pline("%s %s %s %s at %s.", Monnam(magr),
 	      (objects[otemp->otyp].oc_dir & PIERCE) ? "thrusts" : "swings",
-	      mhis(magr), xname(otemp), buf);
+	      mhis(magr), singular(otemp, xname), buf);
 }
 
 /*