Unix Plink: Set O_NONBLOCK on the SIGWINCH pipe.

Unix Plink: Set O_NONBLOCK on the SIGWINCH pipe.

This should avoid the possibility of the SIGWINCH handler's blocking
when trying to write to the pipe.  This could only happen if we'd
somehow received PIPE_BUF SIGWINCHes without reading the pipe, which
would be difficult to achieve.

While we're at it, also set O_NONBLOCK on the reading side of the pipe,
just in case.