implement explicit-exit-notify

[ghost]

       --explicit-exit-notify [n]
              In UDP client mode or point-to-point mode, send  server/peer  an
              exit  notification  if tunnel is restarted or OpenVPN process is
              exited.  In client mode, on exit/restart, this option will  tell
              the  server  to  immediately  close  its  client instance object
              rather than waiting for a timeout.  The n parameter  (default=1)
              controls the maximum number of attempts that the client will try
              to resend the exit notification message.

              In UDP server mode, send RESTART control channel command to con‐
              nected  clients.  The  n  parameter  (default=1) controls client
              behavior. With n = 1 client will attempt  to  reconnect  to  the
              same server, with n = 2 client will advance to the next server.

              OpenVPN  will not send any exit notifications unless this option
              is enabled.

This option can be pushed to the client by the server. The server can also send the notification to the client in case the server restarts or shuts down.

[ghost]

I think it may actually make sense to (when using UDP) always send this exit notify when disconnecting, independent on whether or not the server pushes this. What do you think?

[ghost]

The reason for wanting this is that when the client disconnects the server doesn't know the client is gone and has to wait for the "ping timeout" while keeping a "ghost" connection up for no reason...

[ghost]

It turns out this is only part of the fix. The server can also send an exit-notify to the client when the server is about to shut down. This is important when using UDP, otherwise it will take quite some time for client to find out that it should try to reconnect...