[Nickle] pipe() fixed

[Bart Massey]

Thanks to a suggestion from Keith, I fixed pipe() so that it
will fail when passed a bogus program path.

Not done yet, though :-).  Because the file returned by
pipe() is actually a socketpair(2), and because many apps
won't output until their stdin is closed, we need a way to
close just the write side of the pipe file ala shutdown(2).
Haven't looked carefully, but didn't see anything...

	Bart