pkg_radd improvements

[Justin C. Sherrill]

If you're using 2.3.1 or 2.3.2, pkg_radd should work now to install
packages from pkgsrc-2009Q2.  I finally managed to get a complete set
built.

Also, if you're on a 2.2.x system, pkg_radd will default to
pkgbox.dragonflybsd.org for downloads.  I have a redirectmatch rule now on
pkgbox that will send those requests to avalon, where the same packages
are available but the bandwidth is better.

I've done limited testing of pkg_radd on 2.3.1 and 2.2.1, and it appears
to be working better.  pkg_radd should work to install anything that has
matching binaries on avalon.dragonflybsd.org at this point.  (Tell me if
I'm wrong.)