support systemd notifications

Fixes #1733

vendor/github.com/okzk/sdnotify/notify_linux.go

@@ -0,0 +1,23 @@
+package sdnotify
+
+import (
+	"net"
+	"os"
+)
+
+// SdNotify sends a specified string to the systemd notification socket.
+func SdNotify(state string) error {
+	name := os.Getenv("NOTIFY_SOCKET")
+	if name == "" {
+		return ErrSdNotifyNoSocket
+	}
+
+	conn, err := net.DialUnix("unixgram", nil, &net.UnixAddr{Name: name, Net: "unixgram"})
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	_, err = conn.Write([]byte(state))
+	return err
+}