r4791: used the new talloc type safety macros to make the "void *private"

r4791: used the new talloc type safety macros to make the "void *private"
pointers in the composite code type safe.

This is a bit of an experiement, I'd be interested in comments on
whether we should use this more widely.
(This used to be commit 0e1da827b380998355f75f4ef4f424802059c278)