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/decl.h

@@ -1199,6 +1199,7 @@ struct const_globals {
     const struct obj zeroobj;      /* used to zero out a struct obj */
     const struct monst zeromonst;  /* used to zero out a struct monst */
     const anything zeroany;        /* used to zero out union any */
+    const struct rm zerorm;        /* used to zero out struct rm */
 };
 
 E const struct const_globals cg;

include/display.h

@@ -275,6 +275,9 @@
  *
  * statue       One for each monster.  Count: NUMMONS
  *
+ * unexplored   One for unexplored areas of the map
+ * nothing      Nothing but background
+ *
  * The following are offsets used to convert to and from a glyph.
  */
 #define NUM_ZAP 8 /* number of zap beam types */
@@ -292,10 +295,14 @@
 #define GLYPH_SWALLOW_OFF ((NUM_ZAP << 2) + GLYPH_ZAP_OFF)
 #define GLYPH_WARNING_OFF ((NUMMONS << 3) + GLYPH_SWALLOW_OFF)
 #define GLYPH_STATUE_OFF  (WARNCOUNT + GLYPH_WARNING_OFF)
-#define MAX_GLYPH         (NUMMONS + GLYPH_STATUE_OFF)
+#define GLYPH_UNEXPLORED_OFF (NUMMONS + GLYPH_STATUE_OFF)
+#define GLYPH_NOTHING_OFF (GLYPH_UNEXPLORED_OFF + 1)
+#define MAX_GLYPH         (GLYPH_NOTHING_OFF + 1)
 
 #define NO_GLYPH          MAX_GLYPH
 #define GLYPH_INVISIBLE   GLYPH_INVIS_OFF
+#define GLYPH_UNEXPLORED  GLYPH_UNEXPLORED_OFF
+#define GLYPH_NOTHING     GLYPH_NOTHING_OFF
 
 #define warning_to_glyph(mwarnlev) ((mwarnlev) + GLYPH_WARNING_OFF)
 #define mon_to_glyph(mon, rng)                                      \
@@ -434,5 +441,7 @@
 #define glyph_is_warning(glyph)   \
     ((glyph) >= GLYPH_WARNING_OFF \
      && (glyph) < (GLYPH_WARNING_OFF + WARNCOUNT))
+#define glyph_is_unexplored(glyph) ((glyph) == GLYPH_UNEXPLORED)
+#define glyph_is_nothing(glyph) ((glyph) == GLYPH_NOTHING)
 
 #endif /* DISPLAY_H */

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)

include/rm.h

@@ -264,11 +264,13 @@ struct symparse {
 };
 
 /* misc symbol definitions */
-#define SYM_BOULDER 0
-#define SYM_INVISIBLE 1
-#define SYM_PET_OVERRIDE 2
-#define SYM_HERO_OVERRIDE 3
-#define MAXOTHER 4
+#define SYM_NOTHING 0
+#define SYM_UNEXPLORED 1
+#define SYM_BOULDER 2
+#define SYM_INVISIBLE 3
+#define SYM_PET_OVERRIDE 4
+#define SYM_HERO_OVERRIDE 5
+#define MAXOTHER 6
 
 /* linked list of symsets and their characteristics */
 struct symsetentry {