client.go

during the running of the client it might happen that there's an
actual error for which we need to restart the UDP listener where
openvpn send its traffic, but an error will also be  encountered
in the copy loop when tearing down the connection, we can filter
these types of errors by checking if it is a net.ErrClosed error
type and re-create the UDP listener only when the error is not a
net.ErrClosed