ipa-kdb: Free talloc autofree context when module is closed
    
    Currently kdb5kdc crashes on exit if the ipadb KDB modules is loaded and trusts
    are configured. The reason is the talloc autofree context which get initialised
    during the ndr_push_union_blob() call. On exit the KDC module is unloaded an
    later on atexit() tries to free the context, but all related symbols are
    already unloaded with the module.
    
    This patch frees the talloc autofree context during the cleanup routine of the
    module. Since this is called only at exit and not during normal operations this
    is safe even if other KDC plugins use the talloc autofree context, e.g. via
    some Samba libraries, as well.
    
    Fixes https://fedorahosted.org/freeipa/ticket/3410