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.

tmux.h

@@ -1837,6 +1837,7 @@ struct tty_term {
 #define TERM_SIXEL 0x40
 #define TERM_INVALIDMS 0x80
 #define TERM_NOREPLACE 0x100
+#define TERM_KITTYKEYS 0x200
 	int		 flags;
 
 	LIST_ENTRY(tty_term) entry;
@@ -1918,7 +1919,6 @@ struct tty {
 #define TTY_HAVESYNC 0x10000
 #define TTY_HAVEKKB 0x20000
 #define TTY_KKBPUSHED 0x40000
-#define TTY_KKBSUPPORT 0x80000
 #define TTY_ALL_REQUEST_FLAGS \
 	(TTY_HAVEDA|TTY_HAVEDA2|TTY_HAVEXDA|TTY_HAVESYNC|TTY_HAVEKKB)
 	int		 flags;
@@ -3524,12 +3524,10 @@ void	 input_cancel_requests(struct client *);
 
 /* input-key.c */
 void	 input_key_build(void);
-int	 input_key_pane(struct window_pane *, key_code, struct mouse_event *,
-	     int);
-int	 input_key(struct screen *, struct bufferevent *, key_code, int);
+int	 input_key_pane(struct window_pane *, key_code, struct mouse_event *);
+int	 input_key(struct screen *, struct bufferevent *, key_code);
 int	 input_key_get_mouse(struct screen *, struct mouse_event *, u_int,
 	     u_int, const char **, size_t *);
-int	 input_key_client_supports_extended(struct client *, key_code);
 
 /* input-kitty.c */
 void	 input_kitty_reset(struct screen *);