first module: filter-aaaa

This is a new branch with commit history squashed down from the 50-odd commits that were in the hooks-initial branch.

Known issues which were preliminarily decided to be addressed at a later time:

• After implementing hook modules, add a way to check them with named-checkconf #541 (closed)
• Initial implementation of modules doesn't support multiple instances #547 (closed)
• Hook module interface can be simplified after dropping support for statically linking named #548 (closed)