statusbar: suppress --constantshow when the terminal has just one row

When there is just one row, the text to be edited needs to be shown
there, not some meta information about the cursor position.

This fixes https://savannah.gnu.org/bugs/?60563.

Bug existed since version 2.7.0, since nano allows very flat terminals.

src/nano.c

@@ -2507,7 +2507,8 @@ int main(int argc, char **argv)
 #endif
 		/* Update the displayed current cursor position only when there
 		 * is no message and no keys are waiting in the input buffer. */
-		if (ISSET(CONSTANT_SHOW) && lastmessage == VACUUM && get_key_buffer_len() == 0)
+		if (ISSET(CONSTANT_SHOW) && lastmessage == VACUUM && LINES > 1 &&
+										get_key_buffer_len() == 0)
 			report_cursor_position();
 
 		as_an_at = TRUE;

src/winio.c

@@ -1683,7 +1683,7 @@ void check_statusblank(void)
 	statusblank--;
 
 	/* When editing and 'constantshow' is active, skip the blanking. */
-	if (currmenu == MMAIN && ISSET(CONSTANT_SHOW))
+	if (currmenu == MMAIN && ISSET(CONSTANT_SHOW) && LINES > 1)
 		return;
 
 	if (statusblank == 0)