Return whether the file is readable or not in the DAV permissions

Until now it was safe to assume that every file was readable by its
owner, so there was no need to return whether the file was readable or
not. However, with the introduction of end to end encryption that is no
longer the case, and it is now necessary to explicitly provide that
information.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>