Nonblocking accept

Thanks to the Matz and colleagues for adding the *_nonblock functions. They
have been a huge help. I've already gotten some big performance improvements
in the EventMachine event-processing library. I thought it was a little
strange that accept_nonblock is not available for TCPServer (because it
inherits from IO rather than Socket) but it wasn't hard to work around that.
(Just create a Socket object and call bind and listen on it myself.) The
nonblock functions seem to work well with files and unix-domain sockets. I
haven't tested datagrams yet but will do so shortly.

Thanks again, team.