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This CL updates CL 7511043;
- adds new test cases for both UDPConn and UnixConn,
- makes sure unnamed UnixAddr handling,
- replaces t.Errorf with t.Fatalf in sockname related test cases.

Fixes #3721 (again).
Fixes #3838 (again).

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7627048