- in multi, never modify _default value
- _reset slave when _reset master
- if append None in master with callback return not a list, replace None by the value
- list support in slave
* _cfgimpl_ => _impl_
* optimpl_ => impl_
* properties/permissives are now set/frozenset
* validation raise ValueError if not valid, didn't return anything otherwise
* consistencies are now validate in setting and when deleting value
* ip/network with netmask consistency now works
* DomainnameOption now works
* if no validation, don't set cache for value
* symlinkoption: remove path (not used)