[dev] Filters, Vacation, etc. as service, not driver

[marcus@riboflavin.net]

There have been suggestions of merging sork, ingo, etc., to provide
one location for vacations, forwards, etc., as well as having IMP use
a filters driver structure.  Instead, why don't we make them api
services controlled by the registry.  If I like Ingo's filters, but
not the vacation stuff, I can define mail/vacation as sork's vacation
module and mail/filters as ingo.  For filters, if no service is
registered, IMP can fall-back on its internals.  That way if you want
to play with Ingo but just not define the filter service.

I'm not sure I've expressed the idea totally clearly, but hopefully
you get the general idea.  I don't have time for implementing it, and
if I didn't send a message I would forget about it.  G'night all.