Be more selective about legacy retrofits on package read

Be more selective about legacy retrofits on package read

- Missing RPMTAG_DIRNAME can mean either a package with uncompressed
  filenames OR a new package with no files, in which case the retrofits
  should not be performed. Also a newer package can be built with
  --nodirtokens, requiring filelist compression but no other retrofits.
- Test for immutable region presence for more accurate v3 package
  detection, if not present do a full retrofit. For others, explicitly
  test for OLDFILENAMES presence and only compress the filelist if needed.e