src/usocket: Do not setblocking on destroy;

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
src/usocket: Don't unset/set O_NONBLOCK around listen() or shutdown() calls.

It doesn't effect them.
Not true on windows
Merge pull request #108 from daurnimator/master

src/usocket: Do not setblocking on destroy, shutdown, and listen.