FUSE: add an unmount command

We need a user story which is better than killall python for unmouonting the FUSE VFS when swh fuse mount has not been invoked with --foreground. (Unless FUSE has a general solution for this? does /usr/bin/umount just work out of the box on a FUSE mount point, maybe? I couldn't experimentally determine this at the moment due to #2699 (closed).)

The most straightforward why to go about this would be adding a swh fuse umount command. And we can store PID information about the FUSE daemon to kill in a virtual PID file within the mount point itself.

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Migrated from T2702 (view on Phabricator)