the pktcache introduces two cache tags PKT/SEC for
basic/DNSSEC responses and stores negative answers in
the cache

previously cryptolib random function was used to generate message id,
this works well but it is slow especially when the entropy is low,
replaced with cryptographically safe prng ISAAC

the ccan directory is going to be used in the future, as it's include
structure makes it easy to embed C snippets instead of reimplementing
them

interface split to ‘map’ and ‘set’
added documentation
array: added array_tail() operation

the build system now doesn't always relink targets