Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

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.

Build and install the atf tests.

Reviewed by: freebsd-testing
Approved by: rpaulo (mentor)

Add ATF to the build. This is may be a bit rought around the egdes,
but committing it helps to get everyone on the same page and makes
sure we make progress.

Tinderbox breakages that are the result of this commit are entirely
the committer's fault -- in other words: buildworld testing on amd64
only.

Credits follow:

Submitted by: Garrett Cooper <yanegomi@gmail.com>
Sponsored by: Isilon Systems
Based on work by: keramida@
Thanks to: gnn@, mdf@, mlaier@, sjg@
Special thanks to: keramida@