net/http: deflake BenchmarkClient and its use of a fixed port for testing

net/http: deflake BenchmarkClient and its use of a fixed port for testing

Let the kernel pick a port for testing, and have the server in the
child process tell the parent (benchmarking) process the port that
was selected.

Fixes flakes like seen in https://golang.org/cl/27050 (and previously)

Change-Id: Ia2b705dc4152f70e0a5725015bdae09984d09d53
Reviewed-on: https://go-review.googlesource.com/27051
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>