RFC: conf-update.d hook dir for dispatch-conf

Portage's dispatch-conf does historically only support RCS for
configuration file archival. There are currently two unresolved
feature requests to extend dispatch-conf support for further
configuration file management tools:

- bug #260623 git support for dispatch-conf [1]
- bug #698316 etckeeper support for dispatch-conf [2]

Extending dispatch-conf by a hook directory, which can be used by
configuration file management tools to receive notifications about
dispatch-conf updating a configuration file, appears to be flexible
approach to satisfy those feature requests (or at least lay the
groundwork for them).

Attached is a patch with a prototypical implementation for
conf-update.d hooks. I am curious whether or not you deem this useful
enough to get into dispatch-conf. If my suggested approach is
considered sensible, then I am happy to continue the work on it. For
example, I want to write a unit test for this. But I am not sure if
portage's test framework already provides the necessary functionality
to create test scenarios for the conf-update.d hooks. Any pointers and
further feedback would be much appreciated.

- Florian

1: https://bugs.gentoo.org/260623
2: https://bugs.gentoo.org/698316