Previously, nm_active_connection_authorize() accepts two user-data
pointers for convenience.

nm_active_connection_authorize() has three callers. One only requires
one user-data, one passes two user-data pointers, and one requires
three pointer.

Also, the way how the third passes the user data (via
g_object_set_qdata_full()) is not great.

Let's only use one user-data pointer. We commonly do that, and it's easy
enough to allocate a buffer to pack multiple pointers together.