bsdinstall: Default to pkgbase if base packages are present

[emaste]

Summary

As of commit 62d18f8c4c10 ("release: Add -DPKGBASE option to include
pkgbase packages") we can include base system packages on the install
media instead of dist tarballs.

Set the default for the traditional/pkgbase question to match the
artifact type included in the install image.

[emaste]

Fixup -- there is no --default-yes in bsddialog and the options were swapped.

[jrm]