gmain: Add private API to create Unix child watch that uses waitid()

This avoids collecting the zombie child, which means that the PID
can't be reused.  This prevents possible race conditions that might
occur were one to send e.g. SIGTERM to a child.

This race condition has always existed due to the way we called
waitpid() for the app, but the window was widened when we moved the
waitpid() calls into a separate thread.

If waitid() isn't available, we return NULL, and consumers of this
private API (namely, GSubprocess) will need to handle that.

https://bugzilla.gnome.org/show_bug.cgi?id=672102