Unix write pipe doesn't accept disconnection when idle

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Now _UnixWritePipeTransport registers itself in poller only if it has pending 
writes.

So when there are no such writes transport did not catch disconnects.

Now eventloop has no way to register ERROR event for file descriptor.

Should we add EVENT_ERROR type and add_error/remove_error functions to 
AbstractEventLoop?

BTW EpollSelector should to process EPOLLHUP as well (always adding this flag 
by default makes sense).

This issue is blocker for subprocess support in tulip.

Original issue reported on code.google.com by andrew.s...@gmail.com on 14 Apr 2013 at 3:07