http: add support for selecting SSL backends at runtime

The Pull Request at curl/curl#1601 adds support
for choosing the SSL backend at runtime to cURL, and will hopefully be
merged before version 7.56.0 comes out.

Git for Windows will ship with those patches backported to 7.54.1 (and
come August 9th, 2017, 7.55.0 and later).

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 because Secure Channel ("schannel") is the native
Windows 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.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config.txt

@@ -1948,6 +1948,11 @@ http.sslCAPath::
 	with when fetching or pushing over HTTPS. Can be overridden
 	by the `GIT_SSL_CAPATH` environment variable.
 
+http.sslBackend::
+	Name of the SSL backend to use (e.g. "openssl" or "schannel").
+	This option is ignored if cURL lacks support for choosing the SSL
+	backend at runtime.
+
 http.pinnedpubkey::
 	Public key of the https service. It may either be the filename of
 	a PEM or DER encoded public key file or a string starting with

http.c

@@ -291,6 +291,29 @@ static int http_options(const char *var, const char *value, void *cb)
 		curl_ssl_try = git_config_bool(var, value);
 		return 0;
 	}
+#if LIBCURL_VERSION_NUM >= 0x073800 || \
+		defined(CURL_WITH_EXPERIMENTAL_SSL_BACKEND_SUPPORT)
+	if (!strcmp("http.sslbackend", var)) {
+		const curl_ssl_backend **backends;
+		struct strbuf buf = STRBUF_INIT;
+		int i;
+
+		switch (curl_global_sslset(-1, value, &backends)) {
+		case CURLSSLSET_UNKNOWN_BACKEND:
+			strbuf_addf(&buf, _("Unsupported SSL backend '%s'. "
+					    "Supported SSL backends:"), value);
+			for (i = 0; backends[i]; i++)
+				strbuf_addf(&buf, "\n\t%s", backends[i]->name);
+			die(buf.buf);
+		case CURLSSLSET_TOO_LATE:
+			die(_("Could not set SSL backend to '%s': already set"),
+			    value);
+		case CURLSSLSET_OK:
+			break; /* Okay! */
+		}
+	}
+#endif
+
 	if (!strcmp("http.minsessions", var)) {
 		min_curl_sessions = git_config_int(var, value);
 #ifndef USE_CURL_MULTI