net: use original raddr if getpeername fails

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

src/pkg/net/server_test.go

@@ -402,7 +402,7 @@ func runDatagramConnClient(t *testing.T, net, laddr, taddr string, isEmpty bool)
 			t.Fatalf("Dial(%q, %q) failed: %v", net, taddr, err)
 		}
 	case "unixgram":
-		c, err = DialUnix(net, &UnixAddr{laddr, net}, &UnixAddr{taddr, net})
+		c, err = DialUnix(net, &UnixAddr{Name: laddr, Net: net}, &UnixAddr{Name: taddr, Net: net})
 		if err != nil {
 			t.Fatalf("DialUnix(%q, {%q, %q}) failed: %v", net, laddr, taddr, err)
 		}