Fix explicit directory %attr() when %defattr() is active (RhBug:481875)

Fix explicit directory %attr() when %defattr() is active (RhBug:481875)

- parseForAttr() doesn't know whether it's dealing with a directory or
  a file, so it can't know which defaults it should use.
  Move all the decision making on which of the explicit/default/implicit
  attributes into addFile() where we do know what kind of entry we're
  dealing with, and only parse in parseForAttr().
- backported from commit 09efbe9656bed123dcfb240a7d94906b981a65d1