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.

OpenSSL version 1.1 has broken its API in quite a few ways. To avoid
having to use ifdef's everywhere, we have implemented the BIO functions
added in version 1.1 ourselves in case we are using the legacy API. We
were implementing them in the header file, though, which doesn't make a
lot of sense, since these functions are only ever being used the the
openssl stream implementation.

Move these functions to the implementation file and mark them static.

LibreSSL 2.7 adds OpenSSL 1.1 API

Signed-off-by: Bernard Spil <brnrd@FreeBSD.org>