net: support all PacketConn and Conn returned by Resolver.Dial
Allow the Resolver.Dial func to return instances of Conn other than
*TCPConn and *UDPConn. If the Conn is also a PacketConn, assume DNS
messages transmitted over the Conn adhere to section 4.2.1. "UDP usage".
Otherwise, follow section 4.2.2. "TCP usage".
Provides a hook mechanism so that DNS queries generated by the net
package may be answered or modified before being sent to over the
network.