detect/prefilter: add post-match 'prefilter' engine

detect/prefilter: add post-match 'prefilter' engine

Add support for special post-match engines. This allows a rule to enable
other rules when it matches.

Implementation is similar to prefilter engines, however prefilter
engines run before individual rules while this post-match engine runs
after and individual rule match. It will then add the new rules to the
existing rule list.