fix #H4144 - rejecting named monster's own name

fix #H4144 - rejecting named monster's own name

Some monsters can't be named, but if the user tried to assign them a
name that matched what they were already called, the rejection message
could be silly.  Reported case was "I'm Izchak, not Izchak!".  The fix
is more general than just for shopkeepers, although their reject
message was silliest when complaining about the name already in use.
For the cited case, feedback will now be 'He is already called Izchak.'