kr_bitcmp: adjust semantics -> memcmp
Motivation: allow bits=0 and consequently 0.0.0.0/0 matches in view and renumber modules. https://gitter.im/CZ-NIC/knot-resolver?at=58c940c721d548df2cdfda5e
We shouldn't mix up error codes with valid results from memcmp; let's just segfault if someone passes a NULL, just as memcmp() itself.