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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:...