Sundry fixes for DOS 16-color VGA mode

To test 16-color mode, specify OPTIONS=video:vga explicitly;
autodetect will choose a VESA mode if it can.

* Draw tiles correctly when redrawing from panning or from changing
  the map mode among text, tiles and overview. Previously, this would
  draw everything with the tile at the hero's position.

* Draw corridor walls with the stone tile.

* Map the statue colors to the nearest neutral tone among the main
  16 colors. This mainly affects altars and female cats.

* Fix the code that shows statues as the generic statue tile. This
  code could be deleted, but the statues don't draw with the full
  range of gray tones.

* Document the option OPTIONS=video:vesa.

sys/msdos/msdoshlp.txt

@@ -47,7 +47,7 @@ OPTIONS=VIDEO
 
 		ie: OPTIONS=video:autodetect
 
-		Possible values are: AUTODETECT, DEFAULT, VGA
+		Possible values are: AUTODETECT, DEFAULT, VGA, VESA
 
 		AUTODETECT Checks for a supported hi-res video
                            adaptor, and if it detects one, NetHack 
@@ -62,6 +62,9 @@ OPTIONS=VIDEO
                            potential to cause machine lock-ups if 
                            the specified video hardware is not present.	
 
+		VESA       Forces use of VESA specific video routines.
+                           Reverts to TTY mode if no VESA BIOS is found.
+
 OPTIONS=VIDEOSHADES
 		(defaults.nh only)
 		Players may wish to add this option because one of their