Move atf-sh from /usr/bin/ to /usr/libexec/

In r266650, we made libatf-c and libatf-c++ private libraries so that no
components outside of the source tree could unintendedly depend on them.

This change does the same for the "atf-sh library" by moving the atf-sh
interpreter from its public location in /usr/bin/ to the private location
in /usr/libexec/. Our build system will ensure that our own test programs
use the right binary, but users won't be able to depend on atf-sh by
"mistake".

Committing this now to ride the UPDATING notice added with r267172 today.