https://sourceforge.net/projects/asio/files/asio/1.5.2%20%28Development%29/
* Cleaned up the handling of errors reported by the `close()` system call. In particular, assume that most operating systems won't have `close()` fail with `EWOULDBLOCK`, but if it does then set the blocking mode and restart the call. If any other error occurs, assume the descriptor is closed.

Previous versions uncoditionally made sockets blocking on close(), which
was propagated to other processes that share the same socket.
It (could) subsequently cause a hangup-like situation.