http: add support for selecting SSL backends at runtime

http: add support for selecting SSL backends at runtime

As of version 7.56.0, curl supports being compiled with multiple SSL
backends.

This patch adds the Git side of that feature: by setting http.sslBackend
to "openssl" or "schannel", Git for Windows can now choose the SSL
backend at runtime.

This comes in handy on Windows because Secure Channel ("schannel") is
the native solution, accessing the Windows Credential Store, thereby
allowing for enterprise-wide management of certificates. For historical
reasons, Git for Windows needs to support OpenSSL still, as it has
previously been the only supported SSL backend in Git for Windows for
almost a decade.

The patch has been carried in Git for Windows for over a year, and is
considered mature.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>