bgpd: add flowspec utilities to decode, convert fs nlri

bgpd: add flowspec utilities to decode, convert fs nlri

The FS nlri is depicted so as to be able to be in readable format,
either by human, or by some other ( remote daemon ?).
This work is a derived work from [0]. Initially done for validation
only, this work is extended.
The FS NLRI is able to decode addresses, numbers ( protocols, ports,
tcp values) combined ( or not) with operators. This makes possible
to have a NLRI entry for a set of ports, and/or for an other set of
value of port.
This implementation mainly brings the API visible. The API should be
consistent across the various usages.

[0] https://github.com/chinatelecom-sdn-group/quagga_flowspec/

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: jaydom <chinatelecom-sdn-group@github.com>