allow subconnection to be rue-used as vpn

On a connection it can be specified to use an existing VPN. However, the selectable sessions are only main connections, and not one of their sub-connections.

Context for a mysqldb:
- main connection: host
- sub connection: port forward to connect on localhost (should only be active when needed)
- sub connection: other connection that requires above subconnection to connect
- sub connection: yet other connection that requires first subconnection to connect
- sub connect: ssh shell, does not require the port forward