bgpd, lib: support for flow_label flowspec type

bgpd, lib: support for flow_label flowspec type

in ipv6 flowspec, a new type is defined to be able to do filtering rules
based on 20 bits flow label field as depicted in [0]. The change include
the decoding by flowspec, and the addition of a new attribute in policy
routing rule, so that the data is ready to be sent to zebra.
The commit also includes a check on fragment option, since dont fragment
bit does not exist in ipv6, the value should always be set to 0,
otherwise the flowspec rule becomes invalid.

[0] https://tools.ietf.org/html/draft-ietf-idr-flow-spec-v6-09

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>