Add STNOUPGRADE environment variable to prevents autoupgrades

So, I finally managed to get Syncthing to compile and, as celebration, I would like to request this simple feature.

I would like STNOUPGRADE environment variable that, if defined, would prevent syncthing from autoupdating even if `autoUpgradeIntervalH` is not set to 0. This may help anyone who is running synthing as service and especially me, as I need Syncthing-GTK to manage upgrading on Windows.