ENH: add a Pstream::shutdown() method (#1660)

- previously used a Pstream::exit() invoked from the argList
  destructor to handle all MPI shutdown, but this has the unfortunate
  side-effect of using a fixed return value for the program exit.

  Instead use the Pstream::shutdown() method in the destructor and allow
  the normal program exit codes as usual. This means that the
  following code now works as expected.

  ```
  argList args(...);

  if (...)
  {
      InfoErr<< "some error\n";
      return 1;
  }
  ```