Fix quoting with `metaflac' and cleanup

_retag_analyze() and _retag_apply():
  `metaflac' does not accept sending quoted strings inside an array or
  string as part of an argument (multiple arguments specified).  As a
  result, the previous quoting broke it's effectiveness, leading to
  false logs detailing missing tags.

  The quoting is necessary in order to properly handle spaces in tag
  fields (the tag field, not the tag value).  Spaces are a valid
  character for VORBIS COMMENTS fields, though they are extremely rare.

  A result of this fix/refactoring, will make these functions perform a
  bit slower.

_print_progress():
  Minor cleanup and enforce the parent directory is displayed for files
  and directories, if directory called on the command line is '.' or
  './'.