Follow-up to [b65f3c27ce]: Speedup deleting old packages significantly

The problem was that port_is_needed() was called before parallelizing, which
bottlenecked the delete_old_pkg() call. Moving that check into delete_old_pkg()
allows more to run in parallel again and removes the bottleneck.

In a test of bulk -Cf p5.ports, the dependency/delete startup went from
400 seconds to 161 seconds.
jail -cu: Disable CCACHE on 10
Set the foundation for multiple platform support

Poudriere is hardcoded for FreeBSD in many places.  This patchset sets
the foundation for abstraction.  Much more is needed, but this is a
good start to lay the foundation.
Align with trunk, ready to start debugging