If the stream claims to support this feature, we can let the transport
set the proxy.
We also set HTTPPROXYTUNNEL option so curl can create a tunnel through
the proxy which lets us create our own TLS session (if needed).
We currently have gitno for talking over TCP, but this needs to know
about both plaintext and OpenSSL connections and the code has gotten
somewhat messy with ifdefs determining which version of the function
should be called.
In order to clean this up and abstract away the details of sending over
the different types of streams, we can instead use an interface and
stack stream implementations.
We may not be able to use the stackability with all streams, but we
are definitely be able to use the abstraction which is currently spread
between different bits of gitno.
