fix #H2671 - feedback when entering untended shop while blind

From a bug report, you get the message "this shop
appears to be deserted" when entering a shop whose shopkeeper is not there,
even if you can't see at the time.  Change it to be
 'this shop is deserted'
     when you can see that the shop is empty,
 'this shop is untended'
     when there's no shopkeeper but you can see some other monster(s) inside,
 'this shop seems to be deserted'
     when you can see but there is a monster that you can't see (probably a
     hidden mimic, but possibly an invisible monster or one standing behind
     a boulder that's ended up in the shop), or
 'this shop seems to be untended'
     when you can't see (and don't have active telepathy or persistent
     monster detection).

doc/fixes35.0

@@ -844,6 +844,7 @@ very fast hero would sometimes take two consecutive moves with very fast
 	monsters then getting two moves, instead of interleaving the activity
 when a monster zapped by polymorph drops inventory because of its new form,
 	don't let that same zap hit the dropped item(s)
+entering an untended shop while blind gave an inappropriate message
 
 
 Platform- and/or Interface-Specific Fixes