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

Milton Miller <miltonm@bga.com> and Junfeng Yang <yjf@stanford.edu> point
out that we hand a kernel address to sys_wait4 for the status pointer.
This is true, but since we don't have a SIGCHLD handler, it never gets that
far.  Use NULL, and document the fact.