waitid
__weak_reference(__waitid, waitid);
int waitid(idtype_t, id_t, siginfo_t *, int);
vm_object_pip_wait(vm_object_t object, char *waitid)
refcount_wait(&object->paging_in_progress, waitid);
if (waitid(P_PID, child, &cret, WEXITED) < 0) {