Add a terminal feature for enable/disable extended keys (supported by

xterm and mintty) and add an option to make tmux send it. Only forward
extended keys if the application has requested them, even though we use
the CSI u sequence and xterm uses CSI 27 ~ - this is what mintty does as
well.

tmux.1

@@ -3225,6 +3225,12 @@ sessions.
 .Op Ic on | off
 .Xc
 If enabled, the server will exit when there are no attached clients.
+.It Xo Ic extended-keys
+.Op Ic on | off
+.Xc
+When enabled, extended keys are requested from the terminal and if supported
+are recognised by
+.Nm .
 .It Xo Ic focus-events
 .Op Ic on | off
 .Xc
@@ -4054,16 +4060,6 @@ option.
 .Xc
 If this option is set, searches will wrap around the end of the pane contents.
 The default is on.
-.Pp
-.It Xo Ic xterm-keys
-.Op Ic on | off
-.Xc
-If this option is set,
-.Nm
-will generate
-.Xr xterm 1 -style
-function key sequences; these have a number included to indicate modifiers such
-as Shift, Alt or Ctrl.
 .El
 .Pp
 Available pane options are:
@@ -5754,6 +5750,8 @@ Disable and enable bracketed paste.
 These are set automatically if the
 .Em XT
 capability is present.
+.It Em \&Dseks , \&Eneks
+Disable and enable extended keys.
 .It Em \&Dsfcs , \&Enfcs
 Disable and enable focus reporting.
 These are set automatically if the