Polish the symlink config file comparison a bit

Polish the symlink config file comparison a bit

- Similar treatment as in commit 9f9ed70e2d85b0e8f5e2d9cf9969046034d9ad31,
  pay more attention to what kind of files we're comparing in each case,
  add a little breathing room and extra comments.
- Handle the case where on-disk link is identical to the one in new
  package in which case no backup will be needed, fixing two cases
  of unwanted backups in the test-suite.
  (similar to commit 7a50e9de80ab3c47618dad1e9a5b2d9b79bfc8dc)
- Other than the on-disk vs new package change, probably doesn't change
  anything yet, but prepares us for the next steps.