go.net/internal/nettest: add ProtocolNotSupported function

Moves from go.net/ipv4 pacakge to here, we use this function not only
for ipv4 testing but ipv6 testing.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/169150043

internal/nettest/error_posix.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
+
+package nettest
+
+import (
+	"os"
+	"syscall"
+)
+
+func protocolNotSupported(err error) bool {
+	switch err := err.(type) {
+	case syscall.Errno:
+		switch err {
+		case syscall.EPROTONOSUPPORT, syscall.ENOPROTOOPT:
+			return true
+		}
+	case *os.SyscallError:
+		switch err := err.Err.(type) {
+		case syscall.Errno:
+			switch err {
+			case syscall.EPROTONOSUPPORT, syscall.ENOPROTOOPT:
+				return true
+			}
+		}
+	}
+	return false
+}

internal/nettest/error_stub.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build nacl plan9
+
+package nettest
+
+func protocolNotSupported(err error) bool {
+	return false
+}

internal/nettest/stack.go

@@ -28,3 +28,9 @@ func SupportsIPv6() bool {
 	ln.Close()
 	return true
 }
+
+// ProtocolNotSupported reports whether err is a protocol not
+// supported error.
+func ProtocolNotSupported(err error) bool {
+	return protocolNotSupported(err)
+}