support for OpenBSD, NetBSD, and older versions of OSX

[oconnor663]

waitid can probably be exposed on the BSDs. Here's a naive attempt to do that: rust-lang/libc#520

For OSX, if the warnings are to be believed (https://bugs.python.org/msg167016), there are some versions with broken waitid implementations. If we ever run into those, we can add fake "best efford" implementations based on waitpid?

[oconnor663]

Once try_wait lands in stable, we can consider using the "non-blocking wait in a loop with sleeps" strategy. Doing that with libc::waitpid right now would leave the Child in an invalid state, though, which is a no-no for into_inner.