Implement pipe/pipe2

[tiif]

As mentioned in #3442 (comment), it is fairly easy to implement pipe/pipe2 after #3609. The main difference between socketpair and pipe is socketpair is bidirectional where pipe is unidirectional, other logics should be similar. The usage is also quite high according to the shim wishlist.

[RalfJung]

The usage is also quite high according to #2057.

I suspect most of that is due to uses of Command, which involves constructing a pipe... so adding support for pipe2 will just lead to a slightly nicer error, saying that fork is not supported. But that's still a nice improvement, fork is the actual culprit here and I don't think we'll ever support it.