Signed-off-by: John Nunley <dev@notgull.net>

This commit adds a new backend for the process reaper. Rather than
waiting on a signal, it instead registers the process's pidfd into
async-io and waits on that instead.

I've coded this backend to also allow for other systems to be registered
here as well.

Signed-off-by: John Nunley <dev@notgull.net>

As pidfd isn't available in older versions of Linux that Rust still
supports, this is necessary for running on older Linux. In addition,
signals tests are still kept in CI.

Signed-off-by: John Nunley <dev@notgull.net>