this module allows to rewrite address ranges in answers to different address ranges specified in configuration

This reverts commit 0f9e615b.

previously, debug messages were optional with -DWITH_DEBUG
now the debug messages are built in (unless compiled with -DNDEBUG), but
disabled by default

verbose output can be enabled by '-v' or '--verbose' CLI option
or interactively by 'verbose(true|false)' (or in config)

module can identify clients based on their source address or used TSIG
key

this is a small step for me, but a huge step for resolver

by default it blocks queries to private zones, can be extended to match anything in the query content (i.e. water torture style attacks)

the Redis backend supports UNIX, TCP and custom ports
no real transactions nor iteration, but it’s not needed at the moment
transactions keep replies in a freelist, which is purged on transaction
commit / abort

also some cleanup and version bump to Q2

extracted part about writing layers to lib documentation

The engine is responsible for central managemento of resources and work
distribution. This allows to spawn multiple loops.

The engine configuration / interface is based on Lua 5.1-5.2, including
a RPC and a simple CLI interface. This is going to be the core of configuration
later on.