Homogenize libatf-* version numbers with upstream.

The libatf-* major version numbers in FreeBSD were one version ahead of
upstream because, when atf was first imported into FreeBSD, the upstream
numbers were not respected.  This is just confusing and bound to cause
problems down the road.

Fix this by taking advantage of the fact that libatf-* are now private
and that atf is not yet built by default.  However, and unfortunately, a
clean build is needed for tests to continue working once "make
delete-old-libs" has been run; hence the note in UPDATING.