Make obfsproxy more fork-friendly

PTs need to support a few PT interfaces. For example: a) The managed-mode PT configuration protocol b) SOCKS support (also support for proposal229) c) Extended ORPort (and eventually TransportControlPort) d) Proxy support to connect to proxies

pyptlib can help with (a), but does nothing about the other points (e.g. check #7903 (moved)).

It has been suggested that instead of trying to fit those features in pyptlib, we instead implement them in obfsproxy, and suggest to people to use obfsproxy to developer their PTs.

However, it is the case that obfsproxy is optimized for the "app that supports multiple PTs" use case, and not for the "app that can be forked to support your PT".

If we get more people interested in writing PTs, it might make sense to start designing/fork obfsproxy with that use case in mind.