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 issue 4234.
Fixes issue 4839 (again).
Fixes issue 4866.

R=golang-dev, fullung
CC=golang-dev
https://codereview.appspot.com/7400055