displaying generic objects

Add 17 fake objects to objects[], one for each object class.  All
specific color as gray.  They're grouped at the start--actually near
the start since "strange object" is still objects[0]--rather than
being among the objects for each class.  init_object() knows to start
at [MAXOCLASSES] instead of [0]; other code that loops through every
object might need adjusting.

For potions, non-stone gems, and non-novel/non-Book_of_the_Dead
spellbooks that don't have obj->dknown set, display the corresponding
generic object rather the object itself.  Fixes the longstanding bug
of seeing color for not-yet-seen objects whose primary distinguishing
characteristic is their color.  Walking next to a generic object
while able to see its spot will set dknown and redraw as specific.
It's slightly disconcerting to have objects change as you reach them;
I hope it's just a matter of becoming used to that.  (If there is any
code still changing the hero's location manually instead of using
u_on_newpos(), it should be changed to use that routine.)

Most of the new tiles are just a big rendering of punctuation
characters.  The potion, gem, and spellbook ones could be cloned from
a specific object in their class and then have the color removed.  I
started out that way but wasn't happy with the result.  I'm not
artisticly inclined; hopefully someone else will do better.  Each of
them is preceded by a comment beginning with "#_"; the underscore
isn't required, just being used to make the comments stand out a bit.

Invalidates existing save and bones files.

include/objclass.h

@@ -171,11 +171,12 @@ enum objects_nums {
 };
 
 enum misc_object_nums {
+    NUM_POTIONS    = (LAST_POTION - FIRST_POTION + 1),
+    NUM_REAL_GEMS  = (LAST_REAL_GEM - FIRST_REAL_GEM + 1),
     NUM_GLASS_GEMS = (LAST_GLASS_GEM - FIRST_GLASS_GEM + 1),
-    /* 1st +1: last-first subtraction; 2nd +1: extra empty spl_book[] slot;
-       currently LAST_SPELL includes blank, novel, and DeadBook so this
-       overallocates when used to define spl_book[] array */
-    MAXSPELL = (LAST_SPELL + 1 - FIRST_SPELL + 1),
+    /* LAST_SPELL is SPE_BLANK_PAPER, guaranteeing that spl_book[] will
+       have at least one unused slot at end to be used as a terminator */
+    MAXSPELL       = (LAST_SPELL - FIRST_SPELL + 1),
 };
 
 extern NEARDATA struct objclass objects[NUM_OBJECTS + 1];