Implement timeout for openvpn invocation

Right now the client does not check if the openvpn invocation was successful. We should either

1. do an early poll for abnormal termination.
2. set a counter (signal!) when the openvpn command has been launched, and take action (signal error to user) if the client cannot connect to the management interface after a given constant time.

(from redmine: created on 2013-01-07, closed on 2013-06-23, duplicates #2890 (closed))