Fix select()/kqueue() bug with pipe()s

Fix select()/kqueue() bug with pipe()s

* This causes select() / kqueue() was not returning a writable condition
  on a pipe() whos other end was closed early.

  If a select/kqueue test is performed prior to the other end closing,
  then future calls will later properly detect when the other end closes.

* Should fix numerous niggling issues, including csh sometimes failing
  to recognize early exits of children.

Reported-by: tmunro, deef, bug #3307