Support for individual session idle time locking. May be enabled by turning off

the lock-server option (it is on by default). When this is off, each session
locks when it has been idle for the lock-after-time setting. When on, the
entire server locks when ALL sessions have been idle for their individual
lock-after-time settings.

This replaces one global-only option (lock-after-time) with another
(lock-server), but the default behaviour is usually preferable so there don't
seem to be many alternatives.

Diff/idea largely from Thomas Adam, tweaked by me.

tmux.1

@@ -1253,20 +1253,33 @@ Set the maximum number of lines held in window history.
 This setting applies only to new windows - existing window histories are not
 resized and retain the limit at the point they were created.
 .It Ic lock-after-time Ar number
-Lock the server (like the
-.Ic lock-server
+Lock the session (like the
+.Ic lock-session
 command) after
 .Ar number
-seconds of inactivity.
-The default is off (set to 0).
-This has no effect as a session option; it must be set as a global option using
-.Fl g .
+seconds of inactivity, or the entire server (all sessions) if the
+.Ic lock-server
+option is set.
+The default is not to lock (set to 0).
 .It Ic lock-command Ar command
 Command to run when locking each client.
 The default is to run
 .Xr lock 1
 with
 .Fl np .
+.It Xo Ic lock-server
+.Op Ic on | off
+.Xc
+If this option is
+.Ic on 
+(the default),
+instead of each session locking individually as each has been
+idle for
+.Ic lock-after-time
+, the entire server will lock after
+.Em all
+sessions would have locked.
+This has no effect as a session option; it must be set as a global option.
 .It Ic message-attr Ar attributes
 Set status line message attributes, where
 .Ar attributes