Try to generate fileclass in the tag ext at runtime if missing from header

Try to generate fileclass in the tag ext at runtime if missing from header
- For non-regular files we can easily generate this information based
  on file mode + in case of symlinks, the link target is available
  in the header elsewhere. This also means --fileclass will return
  at least partial data for packages built with the external depgen.
- Clean up fileclassTag() a bit while at it, removing redundant fluff.
- Arguably this "magic" should be done inside rpmfiFClass() instead, but
  that'd require changing the API to return malloced data.