Reset the replay_count upon a successful connection.  It's possible that
we could encounter a situation where we connect successfully but need to
replay a request - for example, a connection and initial request
succeeds without authentication but a subsequent call does require
authentication.  Reset the replay count upon any successful request to
afford subsequent replays room to manuever.

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.

Temporarily disallow SSL connections to a proxy until we can understand
the valgrind warnings when tunneling OpenSSL over OpenSSL.

Only load the proxy configuration during connection; we need this data
when we're going to connect to the server, however we may mutate it
after connection (connecting through a CONNECT proxy means that we
should send requests like normal).  If we reload the proxy configuration
but do not actually reconnect (because we're in a keep-alive session)
then we will reload the proxy configuration that we should have mutated.

Thus, only load the proxy configuration when we know that we're going to
reconnect.

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.

Don't allow servers to send us multiple Content-Type, Content-Length
or Location headers.

We previously used cURL to support HTTP proxies.  Now that we've added
this support natively, we can remove the curl dependency.

The implementations of git_openssl_stream_new and
git_mbedtls_stream_new have callers protected by #ifdefs and
are never called unless compiled in.  There's no need for a
dummy implementation.  Remove them.

Natively support HTTPS connections through proxies by speaking CONNECT
to the proxy and then adding a TLS connection on top of the socket.

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.