Flesh out nacl's fake network system to match how all the other
platforms work: all other systems' SetReadDeadline and
SetWriteDeadline affect currently-blocked read & write calls.
This was documented in golang.org/cl/30164 because it was the status
quo and existing packages relied on it. (notably the net/http package)

And add a test.

Change-Id: I074a1054dcabcedc97b173dad5e827f8babf7cfc
Reviewed-on: https://go-review.googlesource.com/31178
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>