syscall: allow nacl's fake network code to Listen twice on the same address

Noticed from nacl trybot failures on new tests in
https://golang.org/cl/16630

Related earlier fix of mine to nacl's listen code:

  syscall: fix nacl listener to not accept connections once closed
  https://go-review.googlesource.com/15940

Perhaps a better fix (in the future?) would be to remove the listener
from the map at close, but that didn't seem entirely straightforward
last time I looked into it. It's not my code, but it seems that the
map entry continues to have a purpose even after Listener close. (?)

But given that this code is only really used for running tests and the
playground, this seems fine.

Change-Id: I43bfedc57c07f215f4d79c18f588d3650687a48f
Reviewed-on: https://go-review.googlesource.com/16650


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>