fix #H4094 - shopkeeper "it" message

fix #H4094 - shopkeeper "it" message

Most shop messages use shkname() to give the shopkeeper's accurate
name (or hallucinatory substitute) even if he or she can't be seen.
stolen_value() was using mon_nam(), which calls shkname() if the
monster is a shopkeeper who can be seen, but produces "it" when not
seen.  Change it to use shkname() like the rest of the shop routines.
Also, replace Monnam() (quite a few instances) with new Shknam() to
do the same duty when the name is at the start of a sentence.

There was also a very obscure bug where if you could see two
shopkeepers at the same time, you could probe the map one spot at
a time with repeated use of the 'p' command to locate monsters in
general and other shopkeepers in particular.  Very tedious and not
very useful, but now fixed.