Add "grouped sessions" which have independent name, options, current window and

so on but where the linked windows are synchronized (ie creating, killing
windows and so on are mirrored between the sessions). A grouped session may be
created by passing -t to new-session.

Had this around for a while, tested by a couple of people.

tmux.1

@@ -409,6 +409,7 @@ Lock all clients attached to
 .Op Fl d
 .Op Fl n Ar window-name
 .Op Fl s Ar session-name
+.Op Fl t Ar target-session
 .Op Ar command
 .Xc
 .D1 (alias: Ic new )
@@ -425,6 +426,26 @@ are the name of and command to execute in the initial window.
 If run from a terminal, any
 .Xr termios 4
 special characters are saved and used for new windows in the new session.
+.Pp
+If
+.Fl t
+is given, the new session is
+.Em grouped
+with
+.Ar target-session .
+This means they share the same set of windows - all windows from
+.Ar target-session
+are linked to the new session and any subsequent new windows or windows being
+closed are applied to both sessions.
+The current and previous window and any session options remain independent and
+either session may be killed without affecting the other.
+Giving
+.Fl n
+or
+.Ar command
+are invalid if
+.Fl t
+is used.
 .It Ic refresh-client Op Fl t Ar target-client
 .D1 (alias: Ic refresh )
 Refresh the current client if bound to a key, or a single client if one is given