effectively enables/disables usage of given IP protocol
for subrequests (the server can still listen on these)

the key is now stored in a format friendly to prefix search, the values
also contain one more 16bit field to store rank of the data (to be
utilised later)

the selected address is now an array with selection, caller can then send the same query to multiple offered targets

refs #35

example:
local rr = pkt:section(kres.section.ANSWER)[1]
print(kres.rr2str(rr))

example:
local rr = pkt:section(kres.section.ANSWER)
for i = 1, #rr do
	if rr[i].type == kres.type.A then
		print(kres.dname2str(rr[i].owner))
		print(‘rdlen:’, #rr[i].rdata)
	end
end

sets maximum UDP/EDNS payload within <1220, 65535>
the default is max unfragmented UDP packet, but it’s
possible to set it higher if the network supports it to avoid
TCP retransmits

todo: active refresh

DS keys are injected into current set (unmanaged)
DNSKEY keys are in the managed set and their RFC5011 state is tracked

todo:
- implement timers and this AddTime/RemTime
- active refresh
- move to a separate module

example:
$ kdig @a.root-servers.net +short +tcp DNSKEY . > root.key
$ kresd -k root.key

before the cache was disabled by default, but this has led to many user
errors (mine as well). this enables it by default (which is what most
people want anyway)

example:

worker.resolve('cz', kres.type.NS, kres.class.IN, 0,
function (pkt)
	local answer = kres.pkt_t(pkt)
	print (answer:rcode())
end)

config:
trust_anchors.negative = { ‘bad.cz’, ‘here.com’ }

all names below these NTA will not be validated
(unless there is an island of trust below these anchors)

preparations for TA rotation and management
in config:
trust_anchors.file = ‘root.key’
trust_anchors.auto = true // NOTIMPL
trust_anchors.add(‘. IN DS …’) // Manual addition

`kresd -a 127.0.0.1 -a ::1` binds to both addresses

the reason is that it's not actively used since we moved to binary
testing, and it depends on libknot internal api that has changed
also removed several unused libknot internal headers

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)