Adjust the update_pkg_poc method to allow restricting its action to a specified user

This way we can ensure we only orphan the package on branches where the
user is POC even if it's not the case for the specified branch.

Useful for mass-orphan where we orphan all branches at once and thus
the list of branches includes more than just what the user has.
Expose the former_poc optional argument in api_package_orphan
Add unit-test ensure if former_poc is specified we do not orphan a branch that's not his/her
Merge pull request #109 from fedora-infra/fix_orphan

Fix orphan for a specified user