Add support for storing a list of expired packages in the meta

[arrowd]

This is a draft PR that I wanted to show off before proceeding further.

• I generalized group_open_schema() into open_schema().
• group_load() and expired_load() may be generalized too.
• Add a config option that would remove expired packages during pkg upgrade? Or maybe pkg autoremove?
• What will actually pass -e to the pkg repo call? Changes are required on the Poudriere side?
• Bikeshedding? Should this be called "expired_packages" or somehow else?

[bapt]

dedpulication the _load function would be a good idea
I think by default we should only warn about expired in pkg upgrade, and try to remove via pkg autoremove, I would be nice to have a config option (and pkg upgrade argument) which will remove them in the upgrade transaction, but it can be done in a second step.
Regarding the -e yes some change would be needed on the poudriere side, we should perhaps take the opportunity so poudriere calls something in the ports tree, I need to think more about it.
as for the name, I am fine with expired_packages.

[arrowd]

• Deduplicate functions in pkg_repo_create.c
• Write out an UCL file for expired packages along with groups

Now, how do I implement pkg_jobs_test_expired in pkg_jobs.c? The repo structure seems to be an internal knowledge for this layer, so we probably need to add another repo method?