shop credit feedback (trunk only)

shop credit feedback (trunk only)

     Fix the first of a couple of minor things I noticed when viewing
that guy's ttyrec about a hidden inventory item which turned out to be
scrolls of scare monster on the used-up items section of his shop bill.

     Be more precise when a shopkeeper gives the hero credit for dropped
gold.  Instead of just displaying
  N zorkmids are added to your credit.
show either
  You have established N zorkmids credit.
when you had no previous credit, or
  N zorkmids added to your credit; total is now X zorkmids.
when adding to existing credit.