display: suppress spotlight yellow and error red when NO_COLOR is set

This makes nano conform to the https://no-color.org/ idea: suppressing
any color in the output (in the default setup) when NO_COLOR is set in
the environment.

Specifying a color for any interface element will, however, re-enable
also yellow for the spotlight and red for error messages.

src/color.c

@@ -53,6 +53,7 @@ void set_interface_colorpairs(void)
 			}
 			init_pair(index + 1, combo->fg, combo->bg);
 			interface_color_pair[index] = COLOR_PAIR(index + 1) | combo->attributes;
+			rescind_colors = FALSE;
 		} else {
 			if (index == FUNCTION_TAG || index == SCROLL_BAR)
 				interface_color_pair[index] = A_NORMAL;
@@ -72,6 +73,11 @@ void set_interface_colorpairs(void)
 
 		free(color_combo[index]);
 	}
+
+	if (rescind_colors) {
+		interface_color_pair[SPOTLIGHTED] = A_REVERSE;
+		interface_color_pair[ERROR_MESSAGE] = A_REVERSE;
+	}
 }
 
 /* Assign a pair number to each of the foreground/background color combinations

src/global.c

@@ -201,6 +201,8 @@ char *syntaxstr = NULL;
 		/* The color syntax name specified on the command line. */
 bool have_palette = FALSE;
 		/* Whether the colors for the current syntax have been initialized. */
+bool rescind_colors = FALSE;
+		/* Becomes TRUE when NO_COLOR is set in the environment. */
 #endif
 
 int currmenu = MMOST;

src/nano.c

@@ -2087,6 +2087,9 @@ int main(int argc, char **argv)
 	/* If the terminal can do colors, tell ncurses to switch them on. */
 	if (has_colors())
 		start_color();
+
+	/* When requested, suppress the default spotlight and error colors. */
+	rescind_colors = (getenv("NO_COLOR") != NULL);
 #endif
 
 	/* Set up the function and shortcut lists.  This needs to be done

src/prototypes.h

@@ -141,6 +141,7 @@ extern char *alt_speller;
 extern syntaxtype *syntaxes;
 extern char *syntaxstr;
 extern bool have_palette;
+extern bool rescind_colors;
 #endif
 
 extern bool refresh_needed;