Add support for the former_poc argument in the pkgdb2client API
If an user or a poc are specified, restrict the orphaning to this user
Fix unorphan action, it requires the new POC to be specified.

The new poc can be specified either with --user or with --poc, but one
of them should be there.
If it is not, raise an exception and inform the user about it.
Merge pull request #19 from fedora-infra/orphan_user

Restrict orphan to a specific user