The old way to find a port was to listen :0 and then
look at what port it picked, close the listener, and then
immediately try to listen on that port.

On some Windows 7 machines that sequence fails at
the second listen, because the first one is still lingering
in the TCP/IP stack somewhere.  (Ironically, most of these
are used in tests of a "second listen", which in this case
ends up being the third listen.)

Instead of this race, just return the listener from the
function, replacing usableLocalPort+Listen with
usableListenPort.

Fixes #3219.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5769045