ENHANCED_SYMBOLS: store all CLR_ colors specially

The 0x1000000 bit (NH_BASIC_COLOR bit) was used to mark
CLR_BLACK when storing it in u->ucolor. Now, all of the basic CLR_*
colors are stored that way.

The NH_BASIC_COLOR bit indicates that the value in u->ucolor is
not an rgb value, rather it is one of the 0-15 basic NetHack colors.
The window-ports need to strip the NH_BASIC_COLOR bit off before using
it for color changes.

include/color.h

@@ -54,6 +54,8 @@
 #define DRAGON_SILVER CLR_BRIGHT_CYAN
 #define HI_ZAP CLR_BRIGHT_BLUE
 
+#define NH_BASIC_COLOR 0x1000000
+
 enum nhcolortype { no_color, nh_color, rgb_color };
 
 struct nethack_color {