Support exit status for exit/wait

One test case for returning a exit status
Passes usertests, but haven't used it to simplify tests

kernel/sysproc.c

@@ -10,7 +10,10 @@
 uint64
 sys_exit(void)
 {
-  exit();
+  int n;
+  if(argint(0, &n) < 0)
+    return -1;
+  exit(n);
   return 0;  // not reached
 }
 
@@ -29,7 +32,10 @@ sys_fork(void)
 uint64
 sys_wait(void)
 {
-  return wait();
+  uint64 p;
+  if(argaddr(0, &p) < 0)
+    return -1;
+  return wait(p);
 }
 
 uint64