net: delete usableLocalPort from test
net: delete usableLocalPort from test

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 issue 3219.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5769045

glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,d0f/go-zh,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,webfd/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,d0f/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,scirelli/scirelli-go,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,Triskite/willstone-goclone,mhennings/marcohennings-go,webfd/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,Triskite/willstone-goclone,Triskite/willstone-goclone,webfd/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,scirelli/scirelli-go,mhennings/marcohennings-go