MAINTAINERS: Add a general "kernel hardening" section

While many large subsystems related to kernel hardening have their own
distinct MAINTAINERS entries, there are some smaller collections that
don't, but are maintained/reviewed by linux-hardening@vger.kernel.org.
Add a section to capture these, add (or replace defunct) trees that are
now all carried in the hardening tree.

Signed-off-by: Kees Cook <keescook@chromium.org>