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/steal.c

@@ -310,8 +310,7 @@ gotobj:
 		pline("%s tries to %s your %s but gives up.",
 		      Monnam(mtmp), how[rn2(SIZE(how))],
 		      (otmp->owornmask & W_ARMOR) ? equipname(otmp) :
-		       (otmp->otyp == CORPSE) ? corpse_xname(otmp, FALSE) :
-		       xname(otmp));
+		       cxname(otmp));
 		/* the fewer items you have, the less likely the thief
 		   is going to stick around to try again (0) instead of
 		   running away (1) */