Properly refcnt child processes

Parents hold a ref on their children.  We didn't incref for this.  For
forked processes, this was working, since we failed to decref.  Spawn,
which decreffed properly, blew up.

I think the ksched still had a copy of the proc, in this case, and it
tried to run it.  The kref check (can't incref when 0) caught this.