feedback: refuse the --constantshow toggle (M-C) on a one-row terminal

When the terminal has just one row, there is no room to constantly show
the position of the cursor, so do not allow the user to switch it on.

This fixes https://savannah.gnu.org/bugs/?61445.

Bug existed since version 5.8, commit 36ffb5f0.

src/nano.c

@@ -1113,7 +1113,7 @@ void toggle_this(int flag)
 		return;
 
 	if (flag == CONSTANT_SHOW) {
-		if (ISSET(ZERO)) {
+		if (ISSET(ZERO) || LINES == 1) {
 			statusline(AHEM, _("Not possible in barless mode"));
 			TOGGLE(flag);
 		} else if (!ISSET(MINIBAR))