fix: race condition between cleanup and mounting

[patrislav1]

There's a race condition between the cleanup step (i.e. removing old/unused mountpoints with rmdir) and the mount step. Since udev launches multiple instances of the script in parallel, it's possible that one instance tries to mount a device just after another instance has deleted the associated mountpoint. (This is not a theoretical scenario, I had this actually happen on a system booting with multiple SD cards attached).

This patch fixes the race condition with a lock in /var/run, making the script execution sequential.