Functions flow_check_cf_bmk_values, flow_check_cf_value_length,
flow4_validate_cf and flow6_validate_cf are now not built with lib but
with conf to enable for better semantic separation.
During NLRI parsing of IPv6 Flowspec, dst prefix was not properly
extracted from NLRI, therefore a received flow was stored in a different
position in flowspec routing table, and was not reachable by command
'show route <flow>'.
Add proper prefix part accessors to flowspec code and use them from BGP
NLRI parsing code.
Thanks to Alex D. for the bugreport.
