Refactor positional parameter handling

__process_positional_parameters():
  Simplify and clean up, refactoring broken '-j,--jobs' handling.
  Remove redundant single argument bail-outs (ie, --help, --version).

  Reorganize how short and long arguments are handled and remove the
  __aucdtect_spectrogram() functions as it's not currently being used.

  Restore the checking of a user-defined artwork directory before
  extracting artwork from FLACs.

__long_help():
  Enforce local variable declarations.

Documentation:
  Lots of cleanup, most notably, the removal of 'aucdtect_spectrogram'
  sicne we've removed the function it references.