src/usocket: Do not setblocking on destroy;

[daurnimator]

This results in unexpected behaviour if the socket has been dup()d, as O_NONBLOCK is shared.
Close is always 'blocking' anyway (on unix systems)

See wahern/cqueues#13 for an example use case

I have not done the same for windows (src/wsocket.c) as closesocket() can have non-blocking behaviour.
see http://msdn.microsoft.com/en-us/library/windows/desktop/ms737582(v=vs.85).aspx

[daurnimator]

@diegonehab to answer your earlier question about shutdown(), that also has no need to call setblocking(), as it does not have a non-blocking mode (on unix).

I can add that to this Pull Request if you want?

[diegonehab]

Please add to the shutdown modification to the pull request?

[daurnimator]

^^ done. also for listen() which also doesn't care about O_NONBLOCK.

I'm not convinced that was necessary; though it can't harm on the systems I looked at.
destroy was a special case, because it was impossible to avoid (as it's the __gc), and there is no other close() function available.