mount/map: use names for map/unmap for easier use

mount/map: use names for map/unmap for easier use

So user doesn't need to remember which loop devices he has mapped to
what.

systemd unit encoding is used to transform a unique identifier for the
mapped image into a suitable name. The files created in /run/pbs-loopdev
will be named accordingly.

The encoding all happens outside fuse_loop.rs, so the fuse_loop module
does not need to care about encodings - it can always assume a name is a
valid filename.

'unmap' without parameter displays all current mappings. It's
autocompletion handler will list the names of all currently mapped
images for easy selection. Unmap by /dev/loopX or loopdev number is
maintained, as those can be distinguished from mapping names.

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