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/jackpal/gateway/gateway_windows.go

@@ -1,43 +0,0 @@
-package gateway
-
-import (
-	"bytes"
-	"io/ioutil"
-	"net"
-	"os/exec"
-)
-
-func DiscoverGateway() (ip net.IP, err error) {
-	routeCmd := exec.Command("route", "print", "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
-	}
-
-	// 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:")) {
-			ipFields := bytes.Fields(outputLines[idx+2])
-			ip = net.ParseIP(string(ipFields[2]))
-			break
-		}
-	}
-	err = routeCmd.Wait()
-	return
-}