FreeBSD version compatibility for 1.6.x

This is a direct commit, not a cherry-pick, as the libafs build is
different for master and the 1.6 branch.

Instead of needing to always play catch-up, reverse the sense of the
conditional and specifically exclude the FreeBSD 5.x and 6.x versions
so that new versions will automatically be in the right conditional.

Change-Id: Ifa1d4004faab8a7d6a94ca8d5aeff90e4c51b88e
Reviewed-on: http://gerrit.openafs.org/9143
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>