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/mklev.c

@@ -584,8 +584,6 @@ makevtele()
 void
 clear_level_structures()
 {
-    static struct rm zerorm = { cmap_to_glyph(S_stone),
-                                0, 0, 0, 0, 0, 0, 0, 0, 0 };
     register int x, y;
     register struct rm *lev;
 
@@ -596,7 +594,7 @@ clear_level_structures()
     for (x = 0; x < COLNO; x++) {
         lev = &levl[x][0];
         for (y = 0; y < ROWNO; y++) {
-            *lev++ = zerorm;
+            *lev++ = cg.zerorm;
             /*
              * These used to be '#if MICROPORT_BUG',
              * with use of memset(0) for '#if !MICROPORT_BUG' below,