WIP: Import unreadable files

Description

Certain artifacts stage things which have no read permission. See #684 for an example.

Changes proposed in this merge request:

• Alter _process_list in utils.py so it can read files with no read permission.

There were other previously proposed changes which aren't necessary:

• Alter _process_list so it can do the same job for directories, symlinks and other node types

  • Directories, devices, and fifos are created at the destination without needing to read the source.
  • The permissions set on symlinks are irrelevant under a unix-based OS, and do not need to have their contents read anyway (as above)

• Alter cascache.py so it can hash unreadable files

• Allow CasBasedDirectory to import unreadable files

  • #429 is expected to correct CASCache so it can hash unreadable files. CasBasedDirectory doesn't need to open any files; it just asks the CASCache to hash them with add_object.

This merge request, when approved, will close issue/bug: #684

---