new command '`' to show discoveries for one class (trunk only)

Use the grave accent (back tick) character as the keystroke for a
new command which prompts for an object class and then shows a subset of
the discovered objects list covering just the selected class.  Similar
to the 'I' variant of 'i' for viewing inventory, and mainly useful once
the '\' discoveries list has grown long.

dat/hh

@@ -79,6 +79,7 @@ Z       Zap     cast a spell  (use Y instead of Z if number_pad is -1)
 *               ask for combination of ),[,=,",( all at once
 $       gold    count your gold
 +       spells  list the spells you know; also rearrange them if desired
+`       classkn display known items for one class of objects
 _       travel  move via a shortest-path algorithm to a point on the map 
 .       rest    wait a moment
 ,       pickup  pick up all you can carry