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.
Add --{enable|disable}-maintainer-mode enabled by default

Allowing the distfile in package not to try to regenerated the autotooled
components meaning that the ports will now require
--disable-maintainer-mode
Add picosat code.
Integrate picosat to pkg.
More simple assignment.
Request are now SAT assumptions.
Print problematic assumptions if sat problem was not solved.
Some cleanup.
Do not guess assumptions.
Rework installation candidates checking.
Merge pull request #914 from vstakhov/picosat

Use picosat solver as SAT solver
Do not install unneeded stuff.
Fix pkg_checksum_add_entry.
Do not treat previous checksum versions as invalid.
Add field names to checksummed data too.