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.

src/decl.c

@@ -588,6 +588,7 @@ const struct instance_globals_o g_init_o = {
     UNDEFINED_PTR, /* occupation */
     0, /* occtime */
     UNDEFINED_VALUE, /* otg_temp */
+    NULL, /* otg_otmp */
     NULL, /* occtxt */
     /* symbols.c */
     DUMMY, /* ov_primary_syms */