Accept an enum (`git_stream_t`) during custom stream registration that
indicates whether the registration structure should be used for standard
(non-TLS) streams or TLS streams.

Update the new stream registration API to be `git_stream_register`
which takes a registration structure and a TLS boolean.  This allows
callers to register non-TLS streams as well as TLS streams.

Provide `git_stream_register_tls` that takes just the init callback for
backward compatibliity.

Introduce `git_tls_stream_wrap` which will take an existing `stream`
with an already connected socket and begin speaking TLS on top of it.
This is useful if you've built a connection to a proxy server and you
wish to begin CONNECT over it to tunnel a TLS connection.

Also update the pluggable TLS stream layer so that it can accept a
registration structure that provides an `init` and `wrap` function,
instead of a single initialization function.

This allows the application to use their own TLS stream, regardless of
the capabilities of libgit2 itself.

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.