Give the proxy tests a proxy certificate callback, and allow self-signed
certificates when the `GITTEST_REMOTE_PROXY_SELFSIGNED` environment
variable is set (to anything).  In that case, simply compare the hostname
from the callback to the hostname that we connected to.

Rename credential callback to proxy_cred_cb to match new cert callback.

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.

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

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.

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

For testing, we may wish to use a man-in-the-middle proxy that can
inspect the CONNECT traffic to our test endpoints.  For this, we will
need to accept the proxy's certificate, which will not be valid for the
true endpoint.

Add a new environment variable, GITTEST_REMOTE_SSL_NOVERIFY to disable
https certificate validation for the tests.

The `-Wdocumentation-deprecated-sync` option will warn when there is a
doxygen `\deprecated` tag but there is no corresponding deprecation
attribute on the function.  We want to encourage users to not use
particular APIs by marking them deprecated in the documentation without
necessarily raising a compiler warning by marking an item as deprecated.

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

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.

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.

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

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.

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.