Let launchd handle syncthing restarts.

We are running on OSX, so lets do as the authors recommend and let launchd handle this.

The alternative is to get into fun restart races between launchd and syncthing itself.

     STNORESTART   Do not attempt to restart when requested to, instead just exit.
                   Set this variable when running under a service manager such as
                   runit, launchd, etc.

This is an environment variable we can set and change syncthings behavior.

This PR adds the environment variable to the launchd plist included in the formula.

<3