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.

include/hack.h

@@ -18,6 +18,7 @@
 #define BOLT_LIM 8        /* from this distance ranged attacks will be made */
 #define MAX_CARR_CAP 1000 /* so that boulders can be heavier */
 #define DUMMY { 0 }       /* array initializer, letting [1..N-1] default */
+#define DEF_NOTHING ' '   /* default symbol for NOTHING and UNEXPLORED  */
 
 /* The UNDEFINED macros are used to initialize variables whose
    initialized value is not relied upon.
@@ -93,6 +94,8 @@ enum dismount_types {
 #define MG_BW_LAVA 0x0080  /* 'black & white lava': highlight lava if it
                               can't be distringuished from water by color */
 #define MG_BW_ICE  0x0100  /* similar for ice vs floor */
+#define MG_NOTHING 0x0200  /* char represents GLYPH_NOTHING */
+#define MG_UNEXPL  0x0400  /* char represents GLYPH_UNEXPLORED */
 
 /* sellobj_state() states */
 #define SELL_NORMAL (0)