net: avoid Shutdown during Close

Once we've evicted all the blocked I/O, the ref count
should go to zero quickly, so it should be safe to
postpone the close(2) until then.

Fixes #1898.
Fixes #2116.
Fixes #2122.

R=golang-dev, mikioh.mikioh, bradfitz, fullung, iant
CC=golang-dev
https://golang.org/cl/5649076

src/pkg/net/server_test.go

@@ -83,7 +83,7 @@ func connect(t *testing.T, network, addr string, isEmpty bool) {
 	}
 
 	// Send explicit ending for unixpacket.
-	// Older Linux kernels do stop reads on close.
+	// Older Linux kernels do not stop reads on close.
 	if network == "unixpacket" {
 		fd.Write([]byte("END"))
 	}