Rewrite the fuse interface to the device:

• Remove entirely the usage of interface cloning
• Reimplement the passing of fdata using the devfs_*_cdevpriv interface
• Implement a real refcounting for the fdata objects representing the former check on mnt and drv1 members

This requires that mount_fusefs now passes the filedescriptor number
when trying to mount a specific filesystem instance. Also, now all the
devices will open /dev/fuse specifically and nothing else.

This avoids some nasty races in the device cloning entries reusing from
the cloning list or alternatively a big wastage of memory in situations
where filesystems are frequently unmounted and mounted on long-running
systems.

Reported by: pho
Tested by: pho