Refactor: scheduler: reduce code duplication when displaying resources
    
    Refactor native_output_string() to use GString, for readability and
    maintainability. Refactor common_print() to use it, to reduce duplication and
    ensure displays are consistent.
    
    This makes a couple small changes in how things are shown:
    
    * If pe_print_dev is enabled (a debugging flag not actually used by anything),
      the additional resource fields are shown with the resource flags rather than
      their own parenthesized list.
    
    * The new output model is now consistent with the legacy print model in
      displaying resource flags with commas (not spaces) between them.