We used to return the POLLRDHUP event when polling a write-side of a pipe
whose reader is closed, but Linux returns POLLERR|POLLOUT, so let's do
the same.

Linux's behavior can be explained in that when the reader is closed, a write
will return immediately (and hence POLLOUT) but moreover, will return with
an error (EPIPE). And since a read will also cause an error (EBADF when
reading on the write side of the pipe), a POLLERR makes sense and says
that any operation on this fd will now result in an error.

This patch also adds a test for this case.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>