Add required_shlibs to the list of checksummed fields.

This will make packages that depend on different shared lib version
distinguishable to pkg.
Also add provided_shlibs.
Remove pointless fields.
Add users and groups.
Add Google to the list of contributors.
Fix pkg_checksum_add_entry.
Do not treat previous checksum versions as invalid.
Add field names to checksummed data too.
Add dependencies to the digest.
Slightly fix copyright message.
Fix compatibility check for manifest disgest.
Check digests format when initializing repo.