csi: in-band window resize notifications, private mode 2048

This implements
https://gist.github.com/rockorager/e695fb2924d36b2bcf1fff4a3704bd83,
in-band window resize notifications.

When user enables private mode 2048 (in-band resize
notifications), *always* send current size, even if the mode is
already active.

This ensures applications can rely on getting a reply from the
terminal.

render.c

@@ -4072,6 +4072,8 @@ tiocswinsz(struct terminal *term)
         {
             LOG_ERRNO("TIOCSWINSZ");
         }
+
+        term_send_size_notification(term);
     }
 }