Updated terrain with true color Black option, including suggestion for black unexplored space.

[dennis-roof]

Terrain.txt states 'D' is Black. Actually defines.h:5308 states 'd' is Black and 'D' is Dark Gray. Updated reference, also terrain 'nothing' and 'chasm' to 'd' Black as a formality, since they already use the space character. All other Dark Gray terrain seems right. Maybe unexplored space as a 'D' Dark Gray 'x' character is an intentional Unangband variant decision, but I prefer playing with a 'd' Dark 'x' character. This offers more clarity in dungeon levels on x11, especially in forest dungeon areas such as Barrow-downs. Only changing the color, not the character in case any code requires unexplored space to be character 'x'.

[DGoldDragon28]

Regarding the unexplored space, I will assess the feasibility of adding a play-option of the sort.