Add reverse proxying feature

It could be interesting for IBDNS to act as a proxy for a “real” server behind it.

Instead of causing a lookup in an internal database, a query is proxied to another machine and the response IBDNS receives from it is modified in the same way afterwards.

Interactions with CNAME chasing and (when implemented) additional section processing should be evaluated carefully.

There is probably no need to implement a fully-featured resolver; maybe just copying the query received over the appropriate socket (TCP or UDP) would be sufficient.

The response timeout for the server to which to proxy should be configurable. If any timeout occurs, IBDNS must generate a SERVFAIL response by itself.