There was at least one more special case aside from throwing
(jetisoning items to reduce weight after falling in water) which
have needed the same extra code. This is a more general fix.
- 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
