chunkd on-disk format update

A disruptive chunkd on-disk file format change was just pushed to 
hail.git.  It is a minor format update, but it is incompatible.  chunkd 
v0.8+ objects are now binary-incompatible with the just-released chunkd 
v0.7.

The update adds a magic number and object size to the on-disk object 
header (struct be_fs_obj_hdr), plus some reserved space for future 
additions.

The magic number and object size add easy (and CPU-inexpensive) sanity 
checks to the object I/O code.  The object size field is largely 
redundant, intended mainly for disaster recovery.  However, storing the 
object size enables the file format to support multiple objects inside a 
single POSIX filesystem file.

Hopefully this means implementing an APPEND operation in chunkd is now 
feasible.  But if APPEND falls through, these two new be_fs_obj_hdr 
members retain some utility anyway.

	Jeff