Implement a generic interface to forward resolver queries to the lka

Implement a generic interface to forward resolver queries to the lka
process.  Use it for the reverse lookups required by smtp and mta.

Until now, DNS-related lookups were implemented using ad-hoc IMSGs
between the lka and other processes. It turns out to be confusing and
difficult to maintain/extend.  So we want to replace this with a better
set of IMSGs matching the standard resolver interface.

ok gilles@