This reverts the flutter command to use shlock when flock isn't available. It seems that the mkdir method isn't as reliable as we want. I think that this is because the trap isn't always be executed, which is why I think that shlock uses PIDs to help it be more reliable. Unfortunately, that means that we're back to not working over network shares (which is where things were before I moved to the mkdir method, so not really a regression). I did leave in the mkdir method for platforms that have neither flock nor shlock (which should be very few and far between, but still), so at least we'll do some locking there now.