genskel: Fix issues for C++ bindings v3

genskel: Fix issues for C++ bindings v3

The C++ compiler is too much strict. It doesn't accept
valid C because fields are in the wrong order or missing.

 - afb_verb_t generated structure invert vcbdata and session

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

The signature of the declared function was wrong.

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

This add the declaration of the (pre)initialization and onevent functions.

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

Bug-AGL: SPEC-1806

Change-Id: I1abf8a29f831ecc80458edcd3bd094b2108dc227
Signed-off-by: José Bollo <jose.bollo@iot.bzh>