Again, because we wanted to get this done in time for code freeze, I've
gone ahead and merged. The comment revisions will have to be done later.

One modification: NS_HOOKS_ENABLE is now set for both --with-atf and
--enable-developer.

4772.   [test]          Expanded unit testing framework for libns, using
                        hooks to interrupt query flow and inspect state
                        at specified locations. [RT #46173]

9.12.0