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_darwin.go

@@ -0,0 +1,40 @@
+package gateway
+
+import (
+	"bytes"
+	"io/ioutil"
+	"net"
+	"os/exec"
+)
+
+func DiscoverGateway() (ip net.IP, err error) {
+	routeCmd := exec.Command("/sbin/route", "-n", "get", "0.0.0.0")
+	stdOut, err := routeCmd.StdoutPipe()
+	if err != nil {
+		return
+	}
+	if err = routeCmd.Start(); err != nil {
+		return
+	}
+	output, err := ioutil.ReadAll(stdOut)
+	if err != nil {
+		return
+	}
+
+	// Darwin route out format is always like this:
+	//    route to: default
+	// destination: default
+	//        mask: default
+	//     gateway: 192.168.1.1
+	outputLines := bytes.Split(output, []byte("\n"))
+	for _, line := range outputLines {
+		if bytes.Contains(line, []byte("gateway:")) {
+			gatewayFields := bytes.Fields(line)
+			ip = net.ParseIP(string(gatewayFields[1]))
+			break
+		}
+	}
+
+	err = routeCmd.Wait()
+	return
+}