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