runqueue: Really fix sigchld handling

There are several problems. Firstly, a return value of "None" can mean
there is a C signal handler installed so we need to better handle that
case. signal.SIG_DFL is 0 which equates to false so we also need to
handle that by testing explicitly for None.

Finally, the signal handler *must* call waitpid on all child processes
else it will just get called repeatedly, leading to the hanging behaviour
we've been seeing. The solution is to only error for the worker children,
we warn about any other stray children which we'll have to figure out the
sources of in due course.

Hopefully this patch gets things working again properly though.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>