Fix select(2) exceptfds handling of FIFOs and pipes

Fix select(2) exceptfds handling of FIFOs and pipes

Prevent select(2) from indicating an exceptional condition when the
other end of a FIFO or pipe is closed.

Originally, select(2) returned an exceptfds event only with a pty or
socket that has out-of-band data pending. millert@ says that OpenBSD
diverged from this by accident when poll(2) and select(2) were changed
to use the same backend code in year 2003.

OK millert@