Make atf libraries private

[Julio Merino]

Hello,

I'd like to make the libatf-c and libatf-c++ private, both in CURRENT _and_ in stable/10.

These libraries are not built by default yet.  Only when WITH_TESTS=yes is defined the libraries are built and installed.

Once installed, nothing outside of the base system should use them with one exception.  The exception are a bunch of atf-related ports (lutok, kyua*) that can use these libraries if and only if the TESTS ports option is enabled.

So... I don't think moving the library impacts binary compatibility in any significant way.

Plan: bring atf port up to date, move libraries to the private directory, make tests link to the libraries in the new location and add the old libraries to the obsolete list.

I will move the libraries sometime next week unless there are objections to be discussed.  Or submit earlier if somebody confirms this is OK.

Thanks!