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.

window.c

@@ -2033,8 +2033,7 @@ window_pane_copy_paste(struct window_pane *wp, char *buf, size_t len)
 }
 
 static void
-window_pane_copy_key(struct window_pane *wp, key_code key,
-    int extended_encoding)
+window_pane_copy_key(struct window_pane *wp, key_code key)
 {
 	struct window_pane	*loop;
 
@@ -2045,7 +2044,7 @@ window_pane_copy_key(struct window_pane *wp, key_code key,
 		    (~loop->flags & PANE_INPUTOFF) &&
 		    window_pane_is_visible(loop) &&
 		    options_get_number(loop->options, "synchronize-panes"))
-			input_key_pane(loop, key, NULL, extended_encoding);
+			input_key_pane(loop, key, NULL);
 	}
 }
 
@@ -2073,7 +2072,6 @@ window_pane_key(struct window_pane *wp, struct client *c, struct session *s,
     struct winlink *wl, key_code key, struct mouse_event *m)
 {
 	struct window_mode_entry	*wme;
-	int				 extended_encoding;
 
 	if (KEYC_IS_MOUSE(key) && m == NULL)
 		return (-1);
@@ -2096,14 +2094,13 @@ window_pane_key(struct window_pane *wp, struct client *c, struct session *s,
 	if (wp->fd == -1 || wp->flags & PANE_INPUTOFF)
 		return (0);
 
-	extended_encoding = input_key_client_supports_extended(c, key);
-	if (input_key_pane(wp, key, m, extended_encoding) != 0)
+	if (input_key_pane(wp, key, m) != 0)
 		return (-1);
 
 	if (KEYC_IS_MOUSE(key))
 		return (0);
 	if (options_get_number(wp->options, "synchronize-panes"))
-		window_pane_copy_key(wp, key, extended_encoding);
+		window_pane_copy_key(wp, key);
 	return (0);
 }