s3backer is a FUSE filesystem that presents a single fixed-size file
backed by Amazon S3 (or a compatible object store).  The file is split
into blocks, each stored as an S3 object.

Typical use is to put a normal filesystem on that file via a vnode
disk (vnconfig) or similar loopback arrangement, so existing filesystem
implementations handle full UNIX semantics while s3backer only deals
with block GET/PUT/DELETE against the object store.

Features include local block caching, encryption/authentication of
block data, compression, read-ahead, and removal of 'empty' (all zero)
blocks from S3.
