vendor: Replace github.com/jackpal/gateway with github.com/calmh/gateway (fixes #3142)

Switch to my forked version which contains a fix for this issue. I'll
track upstream in the future if things update there, and attempt to
contribute back fixes...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3149

vendor/github.com/calmh/gateway/gateway_common.go

@@ -0,0 +1,38 @@
+package gateway
+
+import (
+	"bytes"
+	"errors"
+	"net"
+)
+
+var errNoGateway = errors.New("no gateway found")
+
+func parseRoutePrint(output []byte) (net.IP, error) {
+	// Windows route output format is always like this:
+	// ===========================================================================
+	// Active Routes:
+	// Network Destination        Netmask          Gateway       Interface  Metric
+	//           0.0.0.0          0.0.0.0      192.168.1.1    192.168.1.100     20
+	// ===========================================================================
+	// I'm trying to pick the active route,
+	// then jump 2 lines and pick the third IP
+	// Not using regex because output is quite standard from Windows XP to 8 (NEEDS TESTING)
+	outputLines := bytes.Split(output, []byte("\n"))
+	for idx, line := range outputLines {
+		if bytes.Contains(line, []byte("Active Routes:")) {
+			if len(outputLines) <= idx+2 {
+				return nil, errNoGateway
+			}
+
+			ipFields := bytes.Fields(outputLines[idx+2])
+			if len(ipFields) < 3 {
+				return nil, errNoGateway
+			}
+
+			ip := net.ParseIP(string(ipFields[2]))
+			return ip, nil
+		}
+	}
+	return nil, errNoGateway
+}