Revamp extended keys support to more closely match xterm and support

mode 2 as well as mode 1. From Stanislav Kljuhhin (GitHub issue 4038).

This changes tmux to always request mode 2 from parent terminal, change
to an unambiguous internal representation of keys, and adds an option
(extended-keys-format) to control the format similar to the xterm(1)
formatOtherKeys resource.

tmux.1

@@ -3733,6 +3733,10 @@ Note that aliases are expanded when a command is parsed rather than when it is
 executed, so binding an alias with
 .Ic bind-key
 will bind the expanded form.
+.It Ic copy-command Ar shell-command
+Give the command to pipe to if the
+.Ic copy-pipe
+copy mode command is used without arguments.
 .It Ic default-terminal Ar terminal
 Set the default terminal for new windows created in this session - the
 default value of the
@@ -3746,10 +3750,6 @@ be set to
 .Ql screen ,
 .Ql tmux
 or a derivative of them.
-.It Ic copy-command Ar shell-command
-Give the command to pipe to if the
-.Ic copy-pipe
-copy mode command is used without arguments.
 .It Ic escape-time Ar time
 Set the time in milliseconds for which
 .Nm
@@ -3771,22 +3771,53 @@ If enabled, the server will exit when there are no attached clients.
 .It Xo Ic extended-keys
 .Op Ic on | off | always
 .Xc
-When
-.Ic on
-or
-.Ic always ,
-the escape sequence to enable extended keys is sent to the terminal, if
-.Nm
-knows that it is supported.
-.Nm
-always recognises extended keys itself.
-If this option is
+Controls how modified keys (keys pressed together with Control, Meta, or Shift)
+are reported.
+This is the equivalent of the
+.Ic modifyOtherKeys
+.Xr xterm 1
+resource.
+.Pp
+When set to
 .Ic on ,
-.Nm
-will only forward extended keys to applications when they request them; if
+the program inside the pane can request one of two modes: mode 1 which changes
+the sequence for only keys which lack an existing well-known representation; or
+mode 2 which changes the sequence for all keys.
+When set to
 .Ic always ,
+mode 1 output is forced and the program cannot change it.
+When set to
+.Ic off ,
+this feature is disabled and only standard keys are reported.
+.Pp
 .Nm
-will always forward the keys.
+will always request extended keys itself if the terminal supports them.
+See also the
+.Ic extkeys
+feature for the
+.Ic terminal-features
+option, the
+.Ic extended-keys-format
+option and the
+.Ic pane_key_mode
+variable.
+.It Xo Ic extended-keys-format
+.Op Ic csi-u | xterm
+.Xc
+Selects one of the two possible formats for reporting modified keys to
+applications.
+This is the equivalent of the
+.Ic formatOtherKeys
+.Xr xterm 1
+resource.
+For example, C-S-a will be reported as
+.Ql ^[[27;6;65~
+when set to
+.Ic xterm ,
+and as
+.Ql ^[[65;6u
+when set to
+.Ic csi-u .
 .It Xo Ic focus-events
 .Op Ic on | off
 .Xc
@@ -5512,6 +5543,7 @@ The following variables are available, where appropriate:
 .It Li "pane_in_mode" Ta "" Ta "1 if pane is in a mode"
 .It Li "pane_index" Ta "#P" Ta "Index of pane"
 .It Li "pane_input_off" Ta "" Ta "1 if input to pane is disabled"
+.It Li "pane_key_mode" Ta "" Ta "Extended key reporting mode in this pane"
 .It Li "pane_last" Ta "" Ta "1 if last pane"
 .It Li "pane_left" Ta "" Ta "Left of pane"
 .It Li "pane_marked" Ta "" Ta "1 if this is the marked pane"