lit and unlit corridors

The default symbol for lit and unlit corridors are the same.  This makes
the lit_corridor option a no-op where the defaults are used and also means
that using a light scroll/spell a corridor has no obvious effect.  To
address that, I special-cased the lit corridor symbol and change its color
to bright white when its the same symbol as the unlit corridor symbol (I
didn't change the default color since I thought that made the lit corridor
look strange using the windows console interface).

src/mapglyph.c

@@ -121,6 +121,14 @@ unsigned *ospecial;
 	    else
 		color = NO_COLOR;
 	} else
+#endif
+#ifdef TEXTCOLOR
+	    /* provide a visible difference if normal and lit corridor
+	     * use the same symbol */
+	    if (iflags.use_color &&
+		offset == S_litcorr && ch == showsyms[S_corr])
+		color = CLR_WHITE;
+	    else
 #endif
 	    cmap_color(offset);
     } else if ((offset = (glyph - GLYPH_OBJ_OFF)) >= 0) {	/* object */