Fix numerous nrelease build issues related to the pkgsrc conversion.

Fix numerous nrelease build issues related to the pkgsrc conversion.

* Place packages on pkgbox.dragonflybsd.org in a release-specific
  directory.  Adjust the fetch URLs to point to this box.
* Add sophisticated checks for required distributions and distribution
  sets, output exact and easy to follow instructions on how to install
  required pieces.
* Add make targets to install required pieces that are missing.
* Install specific package versions.
* Include a /usr/local/share/dfuibe_installer/sources.conf file to override
  the copy list and add additional directories that must be copied
  (/usr/freebsd_pkg and /usr/pkg).