Be more careful about skipping shared file/directory creation

Be more careful about skipping shared file/directory creation

- Only skip shared file/dir creation if its actually being created
  by another package. Previously we could've decided to skip an entry
  where the other file was also being skipped for some other reason.
- Ensure %ghost entries always have FA_SKIP on install, previously
  they often were FA_CREATE which makes no sense for %ghost. We dont
  encounter %ghosts at all during install in the FSM as they dont
  exist in the payload, but the file disposition calculations need
  to take them into account now that we're avoiding redundant creates.
- Fixes a regression from commit f7f5f88f9f3d6587e747b034ccb64a3f00ff4e1e
  which unearthed the %ghost FA_CREATE issue, reported here:
  http://lists.fedoraproject.org/pipermail/buildsys/2013-January/004047.html
(cherry picked from commit a712252392eca75443ca45c10a72873cabec7963)