rbd-fuse: Original code from Andreas Bluemle

Signed-off-by: Andreas Bluemle <andreas.bluemle@itxperts.de>
rbd-fuse: add simple RBD FUSE client

Currently written in C on FUSE hi-level interfaces, so error reporting
could be better.  No serious work done for performance.  But it's
usable as it stands.

Specify -c <conf> and a mountpoint, and images show up as files in
that mountpoint.  You can create new images; they'll be created
with attributes stored in xattrs:

	user.rbdfuse.imagesize: default 1GB
	user.rbdfuse.imageorder: default 22
	user.rbdfuse.imagefeatures: default 1 (layering)

Images may be truncated or extended by rewriting.  Currently
once an image is opened, it's not closed, so it can't be deleted
or changed outside of the fuse path.

Signed-off-by: Dan Mick <dan.mick@inktank.com>