Making atf-check an internal tool

[Julio Merino <jmmv84%gmail.com@localhost>]

Hello,

At the moment, atf-check is a public tool: it gets installed into
${prefix}/bin/ and it is documented as a tool directly usable by
the end user.

I'd like to make the tool private: install it under libexec/ and
turn it into an implementation detail for the shell interface.
Why?  To make things simpler!

Any objections?

Thanks.