- provide a plain stream() method on messageStream to reduce reliance
  on casting operators and slightly opaque operator()() calls etc

- support alternative stream for messageStream serial output.
  This can be used to support local redirection of output.
  For example,

     refPtr<OFstream> logging;   // or autoPtr, unique_ptr etc

     // Later...
     Info.stream(logging.get())
        << "Detailed output ..." << endl;

  This will use the stdout semantics in the normal case, or allow
  redirection to an output file if a target output stream is defined,
  but still effectively use /dev/null on non-master processes.

  This is mostly the same as this ternary

      (logging ? *logging : Info())

  except that the ternary could be incorrect on sub-processes,
  requires more typing etc.

ENH: use case-relative names of dictionary, IOstream for FatalIOError

- normally yields more easily understandable information