Raise file conflicts on differing permissions (user, group, mode)

Raise file conflicts on differing permissions (user, group, mode)

- Two files (or directories) cannot be correctly shared if their
  permissions differ, even if the content is identical: either
  file will end up having wrong permissions, depending on installation
  order. This means a package can among other things silently
  eg relax permissions of eg security sensitive directory (accidentally
  or intentionallY).
- We now require exact match of user, group and entire file mode
  (previously only the file type part of mode was tested)