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.

doc/Guidebook.mn

@@ -4797,8 +4797,7 @@ s	S_spider	(arachnid or centipede)
 @	S_ss3	(magic shield 3 of 4)
 *	S_ss4	(magic shield 4 of 4)
 \(ha	S_statue_trap	(statue trap)
-\ 	S_stone	(solid rock or unexplored terrain
-\ 	\ 	\ or dark part of a room)
+\ 	S_stone	(solid rock)
 ]	S_strange_obj	(strange object)
 \-	S_sw_bc	(swallow bottom center)
 \\	S_sw_bl	(swallow bottom left)
@@ -4822,6 +4821,7 @@ T	S_troll	(troll)
 |	S_trwall	(wall)
 \-	S_tuwall	(wall)
 U	S_umber	(umber hulk)
+\ 	S_unexplored	(unexplored terrain)
 u	S_unicorn	(unicorn or horse)
 <	S_upladder	(ladder up)
 <	S_upstair	(staircase up)