Fix several problems with the callback interface:
- Automatically registered callbacks (i.e. methods named
exc_callback, pre_callback etc) were registered on every
instantiation.
Fix: Do not register callbacks in __init__; instead return the
method when asked for it.
- The calling code had to distinguish between bound methods and
plain functions by checking the 'im_self' attribute.
Fix: Always return the "default" callback as an unbound method.
Registered callbacks now always take the extra `self` argument,
whether they happen to be bound methods or not.
Calling code now always needs to pass the `self` argument.
- Did not work well with inheritance: due to the fact that Python
looks up missing attributes in superclasses, callbacks could
get attached to a superclass if it was instantiated early enough. *
Fix: Instead of attribute lookup, use a dictionary with class keys.
- The interface included the callback types, which are LDAP-specific.
Fix: Create generic register_callback and get_callback mehods,
move LDAP-specific code to BaseLDAPCommand
Update code that calls the callbacks.
Add tests.
Remove lint exceptions for CallbackInterface.
* https://fedorahosted.org/freeipa/ticket/2674
Replace _call_exc_callbacks with a function wrapper, which will automatically
call exception callbacks when an exception is raised from the function. This
removes the need to specify the function and its arguments twice (once in the
function call itself and once in _call_exc_callbacks).
Add some extra checks to existing exception callbacks.
