Removed compound_copy; backups check existence.

compound_copy actually seemed messier than a straight AL with whatever
actions were desired. It was neither sufficiently flexible nor
sufficiently simple, and is therefore discarded in favor of the more
understandable AL representation.
In order for the newer form of dir copy to work correctly, backups need
to check file existence before doing anything. They should do this
anyway.
Fixes to DirBlock.to_action tests.

To accomodate the new structure of directory actions, many of the tests
had to be rewritten to acommodate action lists. Changes include handling
of mkdir when ugo.is_root returns true, and handling of recursive
copying of files (os.walk instead of 'cp -r').