Be more clear that g_return_if_fail is undefined behaviour

In particular, it is not incorrect to g_return_if_fail (..., FALSE)
in a function returning a "success" gboolean and a GError: "failure to
meet the preconditions is an error" takes precedence over the
GError documentation's guarantee that the error will be set on failure.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=660809
Reviewed-by: Emmanuele Bassi
g_child_watch_source_new: POSIX pid must be positive

If we used a non-positive pid, we'd call waitpid(that_pid, ...)
which is exactly the situation this function can't deal with.

On Windows, GPid is a HANDLE (pointer), so I don't think the same thing
applies.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=723743
Reviewed-by: Ryan Lortie