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.

screen-write.c

@@ -435,9 +435,7 @@ screen_write_reset(struct screen_write_ctx *ctx)
 
 	s->mode = MODE_CURSOR|MODE_WRAP;
 
-	if (options_get_number(global_options, "extended-keys") == 2 &&
-	    options_get_number(global_options, "extended-keys-format") !=
-	    EXTENDED_KEYS_KITTY)
+	if (options_get_number(global_options, "extended-keys") == 2)
 		s->mode = (s->mode & ~EXTENDED_KEY_MODES)|MODE_KEYS_EXTENDED;
 	input_kitty_reset(s);