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

Drop all ATF tools code.

We stopped building the tools in r256365 so there is no need to ship
their code any longer.

Approved by: rpaulo (mentor)

Update notes for imports of atf.

This is because the atf vendor branch now includes a verbatim copy of
the distfile sources. As a result, the list of files to-be-removed from
the contrib/ directory is now more aggressive (and different) and the
upgrade notes now only describe stuff that is specific to the atf import
and is not documented in the Subversion Primer.

Approved by: rpaulo (mentor)

Add ATF update instructions and file exclude list.

Submitted by: Garrett Cooper <yanegomi@gmail.com>