fuse: Implement MUSE: MTD in userspace

MUSE allows implementing a MTD in userspace.
So far userspace has control over mtd_read, mtd_write, mtd_erase,
and mtd_sync.
It can also set the following MTD parameters:
name, flags, site, writesize and erasesize.

That way advanced simulators for many type of flashes
can be implemented in userspace.

Signed-off-by: Richard Weinberger <richard@nod.at>