Use either .config/leap or .config/bitmask

In order to get in the Ubuntu Software Center, we need to make bitmask default to using .config/bitmask to store its local configuration.

If .config/bitmask does not exist, then it should fall back to using .config/leap. Alternately, we could rename .config/leap to .config/bitmask if it is detected.

(from redmine: created on 2015-01-14, closed on 2016-05-04)