vtls: add a MesaLink vtls backend

[ymjing]

Squashed from PR #2874 and rebased on commit 92f9db1

MesaLink is a TLS library written in 100% Rust, a programming
language that guarantees memory safety. This PR adds MesaLink as a vtls
backend for curl.

[ymjing]

Hi @bagder, here are the squashed commits for the MesaLink vtls backend.

The "localhost-hack" for SNI is now for CURLDEBUG-builds only:

curl/lib/vtls/mesalink.c

Lines 204 to 223 in 0e828be

	else {
	#ifdef CURLDEBUG
	/* Check if the hostname is 127.0.0.1 or [::1];
	* otherwise reject because MesaLink always wants a valid DNS Name
	* specified in RFC 5280 Section 7.2 */
	if(strncmp(hostname, "127.0.0.1", 9) == 0
	#ifdef ENABLE_IPV6
	|| strncmp(hostname, "[::1]", 5) == 0
	#endif
	) {
	SSL_set_tlsext_host_name(BACKEND->handle, "localhost");
	}
	else
	#endif
	{
	failf(data,
	"ERROR: MesaLink does not accept an IP address as a hostname\n");
	return CURLE_SSL_CONNECT_ERROR;
	}
	}

[bagder]

👍

[bagder]

Thanks @kevinis. Nice work! Merged now.