Hide the implementation of the uid hash table.

Hide the implementation of the uid hash table.

The commands should not know anything about the details.

So replace the FOR_EACH_USER macro by for_each_admissible_user()
which takes a function pointer and calls the given function for
each admissible user, passing a pointer to the user_info struct
of that user.

This allows to un-export uid_hash_table_size and uid_hash_table.