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.

regress/kitty-keys-input.sh

@@ -12,8 +12,7 @@ TERM=screen
 CONF=$(mktemp)
 TMP=$(mktemp)
 
-printf '%s\n' 'set -g extended-keys on' \
-    'set -g extended-keys-format kitty' >"$CONF"
+printf '%s\n' 'set -g extended-keys on' >"$CONF"
 
 TMUX="$TEST_TMUX -LtestKIA$$ -f$CONF"
 TMUX2="$TEST_TMUX -LtestKIB$$ -f$CONF"
@@ -44,7 +43,9 @@ assert_key()
 
 	$TMUX2 command-prompt -k 'display-message -pl "%%"' >"$TMP" &
 	sleep 0.15
-	$TMUX send-keys $keys
+	# Send raw bytes so the outer tmux does not encode them itself.
+	raw=$(printf '%s' "$keys" | sed -e 's/Escape /\\033/g' -e 's/ //g')
+	$TMUX send-keys -H $(printf "$raw" | od -An -v -t x1)
 	wait
 
 	actual=$(tr -d '[:space:]' <"$TMP")
@@ -62,10 +63,6 @@ if ! wait_for_mode 'Kitty 1'; then
 	exit 1
 fi
 
-# The handshake is done, so stop the outer tmux encoding the raw sequences
-# below as Kitty keys itself.
-$TMUX set-option -g extended-keys-format csi-u
-
 # Modifiers, including the Kitty-only Super and Hyper.
 assert_key 'Escape [97;2u' 'S-a'
 assert_key 'Escape [97;3u' 'M-a'