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.

tty-features.c

@@ -247,6 +247,13 @@ static const struct tty_feature tty_feature_sync = {
 	0
 };
 
+/* Terminal supports the Kitty keyboard protocol. */
+static const struct tty_feature tty_feature_kittykeys = {
+	"kittykeys",
+	NULL,
+	TERM_KITTYKEYS
+};
+
 /* Terminal supports extended keys. */
 static const char *const tty_feature_extkeys_capabilities[] = {
 	"Eneks=\\E[>4;2m",
@@ -399,6 +406,7 @@ static const struct tty_feature *const tty_features[] = {
 	&tty_feature_extkeys,
 	&tty_feature_focus,
 	&tty_feature_ignorefkeys,
+	&tty_feature_kittykeys,
 	&tty_feature_margins,
 	&tty_feature_mouse,
 	&tty_feature_osc7,