doc: improve RRL description



See merge request !463

Setting malformed RDATA will invalidate existing RDATA leading to
use-after-free or double-free later.

This is the initial merge request for zone-api. Changes are quite
simple, mainly moving code from one place to the other and adding
a bunch of functions to deal with it. So far the only advantage is
a bit simpler code in the DDNS code and simpler API for it. The insides
are not so nice yet as it requires further work on the changesets.

when setting key from rdata, a copy of binary data struct is made.
that struct is then resized to accomodate for new key.
if successful, binary data points to freed memory
when pubkey parsing fails, the resized copy is discarded
and key binary data left with dangling pointer
freeing key causes double-free and using it use-after-free

LibFuzzer support

This changeset contains:

- support for compiler sanitizers and sanitizer coverage in configure
- support for LibFuzzer
- fuzzy test for packet based on LibFuzzer
- fix for NAPTR RR parsing (found by LibFuzzer)
- limit for maximal number of RRs in a parsed packet

See merge request !460

found by LibFuzzer

namedb/lmdb: make sure the freelist can always hold db tree index

Disclaimer: It's been a WIP for so long time that I have forgotten about this.

The size-limited LMDB instance may be filled completely to a point where you cannot delete it, as there won't be any free pages left. This code makes sure there are always enough free pages to delete the complete DB.

See merge request !459