obs-outputs: Add support for and use mbedTLS for SSL

This diff adds mbedTLS support to the obs-outputs plugin.  PolarSSL and
mbedTLS have grown so different between 2015-or-so when libRTMP was
written, and now it's no longer feasible to just use the USE_POLARSSL
flag.

This commit adds a WITH_RTMPS tri-state CMake variable (auto/on/off),
set to "Auto" by default.  "Auto" will use RTMPS if mbedTLS is found,
otherwise will disable RTMPS.  "On" will make it require mbedTLS,
otherwise fails configuration, and "Off" disables RTMPS support
altogether.

Closes obsproject/obs-studio#1360

plugins/obs-outputs/rtmp-stream.c

@@ -438,6 +438,15 @@ static void set_output_error(struct rtmp_stream *stream)
 	}
 #endif
 
+	// non platform-specific errors
+	if (!msg) {
+		switch (stream->rtmp.last_error_code) {
+		case -0x2700:
+			msg = obs_module_text("SSLCertVerifyFailed");
+			break;
+		}
+	}
+
 	obs_output_set_last_error(stream->output, msg);
 }