rewrite checkpoint

vendor/github.com/guelfey/go.dbus/transport_unixcred.go

@@ -0,0 +1,22 @@
+// +build !darwin
+
+package dbus
+
+import (
+	"io"
+	"os"
+	"syscall"
+)
+
+func (t *unixTransport) SendNullByte() error {
+	ucred := &syscall.Ucred{Pid: int32(os.Getpid()), Uid: uint32(os.Getuid()), Gid: uint32(os.Getgid())}
+	b := syscall.UnixCredentials(ucred)
+	_, oobn, err := t.UnixConn.WriteMsgUnix([]byte{0}, b, nil)
+	if err != nil {
+		return err
+	}
+	if oobn != len(b) {
+		return io.ErrShortWrite
+	}
+	return nil
+}