net: add benchmarks for network interface identification

Current results on linux/amd64:
BenchmarkInterfaces                      20000             80902 ns/op
BenchmarkInterfaceByIndex                50000             71591 ns/op
BenchmarkInterfaceByName                 20000             79908 ns/op
BenchmarkInterfaceAddrs                   2000            836413 ns/op
BenchmarkInterfacesAndAddrs               5000            605946 ns/op
BenchmarkInterfacesAndMulticastAddrs     10000            169029 ns/op

Update issue 4866.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7368046