For 4k sector disks, the minimum frag size is 4k. For a 2G fs

For 4k sector disks, the minimum frag size is 4k. For a 2G fs
that delivers too few inodes to hold a src tree.  So adjust the
density for partitions on a 4k disk if fragsize and density are not
passed on the command line. This is kind of a hack, since we do not
have a way to signal the desired # of inodes from the install script.
ok kettenis@ krw@