feedback: when reporting an unbound function key, mention its number

This makes it easier to figure out what to put in their nanorc file
when the user wants to bind the key.

src/nano.c

@@ -1283,6 +1283,11 @@ void unbound_key(int code)
 		/* TRANSLATORS: This refers to a sequence of escape codes
 		 * (from the keyboard) that nano does not recognize. */
 		statusline(AHEM, _("Unknown sequence"));
+#ifndef NANO_TINY
+		/* TRANSLATORS: This refers to an unbound function key. */
+	else if (code > KEY_F0 && code < KEY_F0 + 25)
+		statusline(AHEM, _("Unbound key: F%i"), code - KEY_F0);
+#endif
 	else if (code > 0x7F)
 		statusline(AHEM, _("Unbound key"));
 	else if (meta_key) {