Note: DNSResolver does not yet *use* DNSSEC, but rather this commit is preparation for including DNSSEC validation. The function in src/wallet/wallet2.cpp that uses DNSResolver still needs its parameters updated accordingly.
Still need to deal with DNSSEC and optional fields in the TXT record.
IPv4 and IPv6 name resolution working. Unit tests written (and passing). net_node.{h,inl} code modified to use DNS seeds.
