Log Message:
Split the ATF libraries into their own devel/atf-libs package.

This change adds a new atf-libs package that provides the libatf-c,
libatf-c++ and libatf-sh libraries by themselves, without any of the
ATF runtime tools.  The atf package has been modified to only install
the runtime utilities (atf-run and atf-report being the major ones)
and depend on atf-libs.

The purpose of this change is to allow packages that install tests to
depend on a lighter-weight package, and to allow the addition of the
upcoming kyua-atf-compat package.  The latter will be a package that
provides atf-run and atf-report replacements based on kyua-cli, and
therefore will conflict with the atf tools (but not the libraries).

While doing this, fix the pkgconfig overrides and ensure that we use
the right version of the ATF libraries given that disabling shared
library building appears to have been broken, possibly for a while.