unify how we start obfsvpn in client mode

In one of our last meetings we discussed that we would like to have one way to start obfsvpn in client mode when we use it to obfuscate openvpn. Currently we use either socks5 or transparent proxying depending on the obfuscation protocol. We lean towards transparent proxying for all transports.

We probably still want to keep the code path for SOCKS5 support, as this might become useful for the implementation of the introducer concept.