changes to rbt.c by passing a NULL pointer for the dns_name_t that
receives the full name of the node that is found. There is probably something
better to do, but this just keeps things compiling.
Changed behaviour so that it is intuitive w.r.t. absolute names.
If 'prefix' is absolute then 'suffix' must be NULL.
If 'prefix' or 'suffix' is absolute then the result is absolute.