Detection, done like sync:

- ESC[?u goes to every VT100-like terminal at attach, whatever the
  options.
- A reply turns on a new kittykeys terminal feature. Users can force
  it on or off with terminal-features kittykeys, including kittykeys@
  to turn it off.
- Changing extended-keys now takes effect immediately for attached
  clients (options.c).

Use, per client, when extended-keys is on: Kitty for clients that have
kittykeys, modifyOtherKeys for clients with extkeys, VT10x otherwise.

Pane side:

- Applications can request either protocol; Kitty wins if one asks for
  both.
- Each application gets what it asked for, whichever client typed the
  key.
- ESC[?u is answered truthfully.
- The per-client gate is gone. window.c and popup.c are simply
  master's versions again.

extended-keys always is back to its master meaning, forcing
modifyOtherKeys mode 1 only. Forcing Kitty on every program would have
sent bash ESC[97;5u for Ctrl-A.

extended-keys-format kitty is still accepted, but now behaves the same
as csi-u.

options.c

@@ -1400,6 +1400,12 @@ options_push_changes(const char *name)
 		TAILQ_FOREACH(loop, &clients, entry)
 			server_client_set_key_table(loop, NULL);
 	}
+	if (strcmp(name, "extended-keys") == 0) {
+		TAILQ_FOREACH(loop, &clients, entry) {
+			if (loop->tty.flags & TTY_STARTED)
+				tty_update_features(&loop->tty);
+		}
+	}
 	if (strcmp(name, "user-keys") == 0) {
 		TAILQ_FOREACH(loop, &clients, entry) {
 			if (loop->tty.flags & TTY_OPENED)