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

include/extern.h

@@ -1316,6 +1316,7 @@ E boolean FDECL(the_unique_obj, (struct obj *obj));
 E char *FDECL(doname, (struct obj *));
 E boolean FDECL(not_fully_identified, (struct obj *));
 E char *FDECL(corpse_xname, (struct obj *,BOOLEAN_P));
+E char *FDECL(cxname, (struct obj *));
 E const char *FDECL(singular, (struct obj *,char *(*)(OBJ_P)));
 E char *FDECL(an, (const char *));
 E char *FDECL(An, (const char *));