r20098: Properly fix issues with create_token_from_username()

r20098: Properly fix issues with create_token_from_username()
reported by James. Ensure that this function allocates
everything on the temporary context except the return
memory. Never call this with a null mem context, and
now use conn->mem_ctx instead in smbd/service.c.
Remove separate free functions for conn->ngroups
and conn->nt_user_token as they are now always
talloc'ed off the conn->mem_ctx. Future optimization
will be to remove conn->mem_ctx and make all objects
pointed to in the conn struct talloc'ed off conn itself.
Easy to free then :-).
Jeremy.