window-slot: Keep current directory open
A autofs mount can be automatically unmounted after a timeout if there is no process holding an open file descriptor from that autofs mount.
If that happens while we are displaying it, we remount it and refresh, as implemented by commit b0e28bc1)
But reloading without user action is disruptive and wasteful. There is also potential for remount to fail for some reason.
Instead, let's hold an open file descriptor for the current location, which stop the timeout unmounting from happening in the first place.
Related: #1514 (closed)