MFC r266650, r267172 (both by jmmv):

r266650:

  Change libatf-c and libatf-c++ to be private libraries.

  We should not be leaking these interfaces to the outside world given
  that it's much easier for third-party components to use the devel/atf
  package from ports.

  As a side-effect, we can also drop the ATF pkgconfig and aclocal files
  from the base system.  Nothing in the base system needs these, and it
  was quite ugly to have to get them installed only so that a few ports
  could build.  The offending ports have been fixed to depend on
  devel/atf explicitly.

  Reviewed by:  bapt

r267172:

  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.

Phabric: D701
Approved by: jmmv (maintainer, mentor)