* Splits into three server tests.
  - TestStreamConnServer for tcp, tcp4, tcp6 and unix networks
  - TestSeqpacketConnServer for unixpacket networks
  - TestDatagramPacketConnServer for udp, udp4, udp6 and unixgram networks
* Adds both PacketConn and Conn test clients to datagram packet conn tests.
* Fixes wildcard listen test cases on dual IP stack platform.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5701066