Improve handling of source-as when an AS_SET is rightmost. In that case

Improve handling of source-as when an AS_SET is rightmost. In that case
use the rightmost AS from the previous AS path segment. As suggested in
rfc6472. Also fix a long standing bug of AS 42 >< 4242 matching 43 - 4241
instead of 1-41 and 4243 and upwards. Last but not least pass the filter_as
struct to as_compare since that will make it easier to implement as-sets.
OK benno@, OK job@ on a previous version