fuse_loop: add automatic cleanup of run files and dangling instances

fuse_loop: add automatic cleanup of run files and dangling instances

A 'map' call will only clean up what it needs, that is only leftover
files or dangling instances of it's own name.

For a full cleanup the user can call 'unmap' without any arguments.

The 'cleanup on error' behaviour of map_loop is removed. It is no longer
needed (since the next call will clean up anyway), and in fact fixes a
bug where trying to map an image twice would result in an error, but
also cleanup the .pid file of the running instance, causing 'unmap' to
fail afterwards.

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