afb-genskel doesn't produce valid C++ skeleton for binding v3

Following issues are seen:

• afb_verb_t generated structure invert vcbdata and session

• wrong verb's callback signature void fun(afb_req) instead of
  void fun(afb_req_t)

• missing init function signation generation, or probably preinit and
  onevent (not tested those ones)

• missing provide_class, require_class, require_api in the
  afb_binding_v3 generated struct