net: fix slow network interface manipulations

This CL reduces unnecessary network facility lookups introduced
by recent changes below.

changeset: 15798:53a4da6a4f4a
net: return correct point-to-point interface address on linux

changeset: 15799:a81ef8e0cc05
net: set up IPv6 scoped addressing zone for network facilities

Also adds a test case for issue	4839.

Benchmark results on linux/amd64, virtual machine:
benchmark                                 old ns/op    new ns/op    delta
BenchmarkInterfaces-2                         80487        80382   -0.13%
BenchmarkInterfaceByIndex-2                   72013        71391   -0.86%
BenchmarkInterfaceByName-2                    79865        80101   +0.30%
BenchmarkInterfaceAddrs-2                     42071       829677  +1872.09%
BenchmarkInterfacesAndAddrs-2                 35016       607622  +1635.27%
BenchmarkInterfacesAndMulticastAddrs-2       169849       169082   -0.45%
old: 15797:9c3930413c1b, new: tip

Benchmark results on linux/amd64, virtual machine:
benchmark                                 old ns/op    new ns/op    delta
BenchmarkInterfaces-2                         80487        81459   +1.21%
BenchmarkInterfaceByIndex-2                   72013        71512   -0.70%
BenchmarkInterfaceByName-2                    79865        80567   +0.88%
BenchmarkInterfaceAddrs-2                     42071       120108  +185.49%
BenchmarkInterfacesAndAddrs-2                 35016        33259   -5.02%
BenchmarkInterfacesAndMulticastAddrs-2       169849        82391  -51.49%
old: 15797:9c3930413c1b, new: tip+CL7400055

Benchmark results on darwin/amd64:
benchmark                                 old ns/op    new ns/op    delta
BenchmarkInterfaces-2                         34402        34231   -0.50%
BenchmarkInterfaceByIndex-2                   13192        12956   -1.79%
BenchmarkInterfaceByName-2                    34791        34388   -1.16%
BenchmarkInterfaceAddrs-2                     36565        63906  +74.77%
BenchmarkInterfacesAndAddrs-2                 17497        31068  +77.56%
BenchmarkInterfacesAndMulticastAddrs-2        25276        66711  +163.93%
old: 15797:9c3930413c1b, new: tip

Benchmark results on darwin/amd64:
benchmark                                 old ns/op    new ns/op    delta
BenchmarkInterfaces-2                         34402        31854   -7.41%
BenchmarkInterfaceByIndex-2                   13192        12950   -1.83%
BenchmarkInterfaceByName-2                    34791        31926   -8.23%
BenchmarkInterfaceAddrs-2                     36565        42144  +15.26%
BenchmarkInterfacesAndAddrs-2                 17497        17329   -0.96%
BenchmarkInterfacesAndMulticastAddrs-2        25276        24870   -1.61%
old: 15797:9c3930413c1b, new: tip+CL7400055

Update #4234.
Fixes #4839 (again).
Fixes #4866.

R=golang-dev, fullung
CC=golang-dev
https://golang.org/cl/7400055
diff --git a/src/pkg/net/interface_linux_test.go b/src/pkg/net/interface_linux_test.go
index f14d1fe..50d3dc6 100644
--- a/src/pkg/net/interface_linux_test.go
+++ b/src/pkg/net/interface_linux_test.go
@@ -4,7 +4,53 @@
 
 package net
 
-import "testing"
+import (
+	"fmt"
+	"os/exec"
+	"testing"
+)
+
+func (ti *testInterface) setBroadcast(suffix int) {
+	ti.name = fmt.Sprintf("gotest%d", suffix)
+	xname, err := exec.LookPath("ip")
+	if err != nil {
+		xname = "ip"
+	}
+	ti.setupCmds = append(ti.setupCmds, &exec.Cmd{
+		Path: xname,
+		Args: []string{"ip", "link", "add", ti.name, "type", "dummy"},
+	})
+	ti.teardownCmds = append(ti.teardownCmds, &exec.Cmd{
+		Path: xname,
+		Args: []string{"ip", "link", "delete", ti.name, "type", "dummy"},
+	})
+}
+
+func (ti *testInterface) setPointToPoint(suffix int, local, remote string) {
+	ti.name = fmt.Sprintf("gotest%d", suffix)
+	ti.local = local
+	ti.remote = remote
+	xname, err := exec.LookPath("ip")
+	if err != nil {
+		xname = "ip"
+	}
+	ti.setupCmds = append(ti.setupCmds, &exec.Cmd{
+		Path: xname,
+		Args: []string{"ip", "tunnel", "add", ti.name, "mode", "gre", "local", local, "remote", remote},
+	})
+	ti.teardownCmds = append(ti.teardownCmds, &exec.Cmd{
+		Path: xname,
+		Args: []string{"ip", "tunnel", "del", ti.name, "mode", "gre", "local", local, "remote", remote},
+	})
+	xname, err = exec.LookPath("ifconfig")
+	if err != nil {
+		xname = "ifconfig"
+	}
+	ti.setupCmds = append(ti.setupCmds, &exec.Cmd{
+		Path: xname,
+		Args: []string{"ifconfig", ti.name, "inet", local, "dstaddr", remote},
+	})
+}
 
 const (
 	numOfTestIPv4MCAddrs = 14