Rework /new_packages, remove mount_packages dance

The /new_packages handling was overly complex, especially for DragonFly.
We only need to worry about two directories as pkgng is only option.

The mount/umount rw/ro dance with /packages seems completely unnecessary
in the presence of /new/packages, so just remove it all.
poudriere.8: -j is required

Reported by:	ale
Fix deadlock

The main culprits were use of arguments in realpath, find, and stat that
don't exist on dragonfly.  A small bulk build now completes.
Trivial merge with trunk branch