unix: read on stream even when UV__POLLHUP set.

This fixes a SmartOS specific issue that happens when reading from
a stream that is the reading end of a pipe that has been closed by
the parent process.

In this case, a UV__POLLHUP event would be set on the stream and would
prevent the event loop from closing it. As a result, the event loop
would think there are stil handles open, and leave the process
hanging.

Fixes #1419.