- LoadAllowedFileStoragePaths: Loads a list of allowed file storage
  paths from a file
- CheckFileStoragePath: Checks a path against the list of allowed paths

The unit test for “utils.IsBelowDir” is updated with cases which weren't
tested before.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>