tweak + ok naddy

This will eventually be used by 'gotadmin pack'. Checksum init and finalization will need to be done by the caller since many objects will be written out in compressed form while we are computing checksums across the entire pack file. ok millert, naddy

also add got_error_prefix_errno2 and got_error_prefix_errno3 which should hopefully all be merged into a single function with variadic args (but can't alloc mem)