From: Rusty Russell <rusty@rustcorp.com.au>

It turns out that run_workqueue never has signal_pending(), since setting
the handler to SIG_IGN means "don't make zombies, I'm ignoring them".  Fix
the comment, don't allow the signal, and remove the unused waitpid loop.

This also allows simpler conversion of workueues to the kthread mechanism,
which uses signals to indicate it's time to stop.