this fixes an issue when nameserver responds with AA=0 and authority
of a CNAME target (which is in current bailiwick)

DNS 0x20 https://tools.ietf.org/html/draft-vixie-dnsext-dns0x20-00
is a way to add more randomness into queries to make spoofing tougher
this implementation provides up to 32 bits of randomness to QNAME,
which is more than enough for most names (it is possible to add a
maximum of 1 bit of entropy per alphanumeric character, so it's not very
efficient with shorter names)

fixes #27

fixes potential cache key oob write

previously a CNAME RR could be merged as a queried type RR, leading to failed cache lookups as ‘expired’

the cache_peek() api was reworked to return an error code instead, from this the caller can tell whether the record is present (but expired) or missing. this save a secondary CNAME lookup in case the original record was just expired

prevents pointless caching of TTL={0,1} records

previously the NS address list was wiped out, this prevents them from being added in the first place

some servers break qname m12n by sending REFUSED, this accepts such answer and requeries with m12n turned off 

the only condition of caching an answer is that it resolves a query and is not already cached
(ok, it must be negative at this point…)