make a distinction between rock and unexplored area

This adds a pair of new glyphs: GLYPH_UNEXPLORED and GLYPH_NOTHING

GLYPH_UNEXPLORED is meant to be the glyph for areas of the map that
haven't been explored yet.

GLYPH_NOTHING is a glyph that represents that which cannot be seen,
for instance the dark part of a room when the dark_room option is
not set.  Since the symbol for stone can now be overridden to
a players choice, it no longer made sense using S_stone for the
dark areas of the room with dark_room off. This allows the same
intended result even if S_stone symbol is mapped to something visible.

GLYPH_UNEXPLORED is what areas of the map get initialized to now
instead of STONE.

This adds a pair of new symbols: S_unexplored and S_nothing.

S_nothing is meant to be left as an unseen character (space) in
order to achieve the intended effect on the display.

S_unexplored is the symbol that is mapped to GLYPH_UNEXPLORED, and
is a distinct symbol from S_stone, even if they are set to the same
character. They don't have to be set to the same character.

Hopefully there are minimal bugs, but it is a deviation from a
fairly long-standing approach so there could be some unintended
glitches that will need repair.

src/read.c

@@ -804,9 +804,7 @@ int howmuch;
         for (zy = 0; zy < ROWNO; zy++)
             if (howmuch & ALL_MAP || rn2(7)) {
                 /* Zonk all memory of this location. */
-                levl[zx][zy].seenv = 0;
-                levl[zx][zy].waslit = 0;
-                levl[zx][zy].glyph = cmap_to_glyph(S_stone);
+                levl[zx][zy] = cg.zerorm;
                 g.lastseentyp[zx][zy] = STONE;
             }
     /* forget overview data for this level */