isofs: convert isofs to use the new mount API

This also renames iso9660_options to isofs_options, for
consistency.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <409e28da-9c19-427a-acfb-78788f0a23b8@redhat.com>