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.

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.

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.

ssl_close uses this boolean to know if SSL_shutdown should be called.
It turns out OpenSSL auto-shutdowns on failure, so if the call to
SSL_connect fails, it will complain about "shutdown while in init",
trampling the original error.

Replace remaining uses of the `git__strtol32` function. While these uses
are all safe as the strings were either sanitized or from a trusted
source, we want to remove `git__strtol32` altogether to avoid future
misuse.

"warning: values of type 'OSStatus' should not be used as format arguments; add an explicit cast to 'int' instead [-Wformat]"

In commit 382ed1e87 (mbedtls: load default CA certificates, 2018-03-29),
the function `git_mbedtls_stream_global_init` was refactored to call out
to `git_mbedtls__set_cert_location` instead of setting up the
certificates itself. The conversion forgot to remove the now-unused
"cacert" variable, which is now only getting declared to be free'd at
the end of the function. Remove it.