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).