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.

doc/fixes36.1

@@ -144,6 +144,10 @@ if a non-cursed spellbook being read becomes cursed and hero recognizes that
 if a non-cursed spellbook being read becomes cursed and hero doesn't notice,
 	don't leave it flagged as in-use (hence to be deleted during the
 	next save/restore cycle) if contact-poison cursed book effect occurs
+stolen_value, used when breaking shop items from outside the shop among other
+	times, could describe the shopkeeper as "it" when not seen
+when two or more shopkeepers were visible at once, 'p' command could be used
+	to probe map for locations of other shopkeepers and arbitrary monsters
 
 
 Platform- and/or Interface-Specific Fixes