Use dark gray color for black glyphs in TTY

This is Michael Deutschmann's use_darkgray -patch.
Adds a boolean option use_darkgray, settable in config file.

This patch has been in use on NAO for years, and I have heard
once someone say their terminal didn't support the dark gray
color.

doc/Guidebook.tex

@@ -2964,6 +2964,9 @@ Specify the preferred height of each tile in a tile capable port.
 \item[\ib{tile\_width}]
 Specify the preferred width of each tile in a tile capable port
 %.lp
+\item[\ib{use\_darkgray}]
+Use bold black instead of blue for black glyphs (TTY only).
+%.lp
 \item[\ib{use\_inverse}]
 NetHack should display inverse when the game specifies it.
 %.lp