I think the memory wouldn't really leak, due to being on a per-request
mempool, but it still seems better to do this properly.

Now that we renamed "debug" to "verbose", the old name didn't seem very
suitable anymore; this new identifier haven't got to master yet.

Split NDEBUG to NDEBUG (asserts) and NOVERBOSELOG (verbose logging); rename all related symbols to use VERBOSE instead of DEBUG

In particular, don't require to call an externally defined function to
find if in --verbose mode or not.  Now it's just an extern bool.
I'm confident the performance impact of not using -DNLOGDEBUG should be
negligible now.

This comes with a small incompatible API+ABI change, but that shouldn't
matter as we've had a couple of those already since the last release.

previously, if no subnet was given (127.0.0.0),
it was treated as 127.0.0.0/0. now it is treated
as full address length, e.g. 127.0.0.0/32

the field length is platform-dependent

this is not going to be backwards compatible change, but it will be the first tagged libknot release sufficient for resolver

* PIE,RELRO+NOW and other security features enabled
* support for both static/dynamic builds with BUILDMODE
* dynamic library is ABI-versioned, starting at 1
* pkg-config file is installed

this is needed to make sure it always compiles with PIC

rdata may be <=64k + 8B on stack which may be source of various mystery
errors later, for example in bindings or stackspace-constricted env

worker can track outbound requests and if N resolutions want the same
subrequest, only one will lead it and others will be notified when it
finishes

this massively reduces number of outbound requests for
slow/unresponsive/low ttl requests

as the libknot packet interface disallows out-of-order packet
writes, authority and additional records must be written after
the answer is complete; records in the rr arrays will be written to final answer during finalization