Enable SIGINT (Ctrl-C) to kill testall instead of leaving it running.

It's quite convoluted, but basically it is a cascade of traps leading
up to killing the actual testall process. Note that it does not kill
any jobs that testall might have created. The hope is that these will
quit on their own in a short time. Basically each level you you want
to kill requires a trap in the parent.
Right, good point. It is a testament to my state of mind after looking at this for too long...