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.
Add missed call to pkg_list_free for PKG_PROVIDES.
Fix pkg_checksum_add_entry.
Do not treat previous checksum versions as invalid.
Add field names to checksummed data too.
Ignore broken packages with the same shlib.

E.g. if package A provides two version of libblah.so then ignore one of
them since we cannot handle this yet.
Merge pull request #919 from ant1/master

Fix issue #916
Merge pull request #921 from gelraen/pkg_reset_provides

Add missed call to pkg_list_free for PKG_PROVIDES.
Add utility to convert string to elf constant.
Add function to compare ABI.
Check shlib arch prior testing it.
Print error if we cannot parse manifest.
Select the newest possible package to satisfy dep.
Another hack to work with broken shlibs.
Ignore shlib duplicates.
Merge pull request #922 from vstakhov/shlibabi

Check shared libraries architecture.