src/usocket: Do not setblocking on destory;

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

See https://github.com/wahern/cqueues/issues/13 for an example use case
Merge pull request #107 from daurnimator/master

src/usocket: Do not setblocking on destory;