fuse_loop: handle unmap on crashed instance

fuse_loop: handle unmap on crashed instance

If a fuse_loop instance dies suddenly (e.g. SIGKILL), the FUSE mount and
loop device assignment are left behind. We can determine this scenario
on specific unmap, when the PID file is either missing or contains a PID
of a non-running process, but the backing file and potentially loop
device are still there.

If that's the case, do an "emergency cleanup", by unassigning the
loopdev, calling 'fusermount -u' and then cleaning any leftover files
manually.

With this in place, pretty much any situation is now recoverable via
only the 'proxmox-backup-client' binary, by either calling 'unmap' with
or without parameters.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>