Turning atf-sh into a private binary

[Julio Merino]

Hello,

Just as we turned libatf-c and libatf-c++ into private libraries with
r266650, I am going to do the same with the "shell library". I.e. I am
going to move atf-sh from /usr/bin into /usr/libexec so that nobody
can depend on atf-sh without intentionally doing so. Effectively, this
turns all of atf into a "private" component of the base system, as it
should be.

The whole point of this change is to prevent people depending on
atf-sh inadvertently: they will still be able to do so by using the
binary from /usr/libexec (just as they can link against libraries in
/usr/lib/private), but then they should understand that they are
depending on internal implementation details that can change at will.

I consider this to be a prerequisite for enabling WITH_TESTS by
default given that moving the binary afterwards would be trickier.

If you want to take a look:

https://phabric.freebsd.org/D187

However, I plan to commit this *today* after I confirm the change
working. The reason is to ride the UPDATING warning that I just
committed with r267172 so that users don't have to clean their object
trees twice in quick succession. Apologies for not giving enough time
to review upfront, and yell if you think this is a horrible idea!

Thanks