revise X11 highlight yn prompting

The three line change I made previously to implement highlighting for
prompts that ask for single-character input was easy and worked well
for a tiles map, but it didn't look very good for a text map.  This
handles both text map and tile map and also adds a configurable
'highlight_prompt' X resource to let the user enable or disable the
feature.  The resource template file (win/X11/NetHack.ad, copied to
$HACKDIR during install) now has it enabled by default.

The highlighting--more specifically, the "lowlighting" when no prompt
is active--still looks bad if the map window has a vertical scrollbar
on left edge.  I don't have any inspiration about how to fix that up.

win/X11/winmap.c

@@ -1,4 +1,4 @@
-/* NetHack 3.6	winmap.c	$NHDT-Date: 1454455161 2016/02/02 23:19:21 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.26 $ */
+/* NetHack 3.6	winmap.c	$NHDT-Date: 1454977918 2016/02/09 00:31:58 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.27 $ */
 /* Copyright (c) Dean Luick, 1992                                 */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -883,6 +883,7 @@ struct xwindow *wp;
         XClearWindow(XtDisplay(wp->w), XtWindow(wp->w));
         set_map_size(wp, COLNO, ROWNO);
         check_cursor_visibility(wp);
+        highlight_yn(TRUE); /* change fg/bg to match map */
     } else if (wp->prevx != wp->cursx || wp->prevy != wp->cursy) {
         register unsigned int x = wp->prevx, y = wp->prevy;