sh: Don't overwrite old exit status if a PID is reused.

Only store exit status for a process if that process has not terminated yet.

Test (slow):
  exit 7 & p1=$!; until exit 8 & p2=$!; [ "$p1" = "$p2" ]; do wait "$p2";
  done; sleep 0.1; wait %1; echo $?
should write "7".