bgpd: support for bgp ipv6 ext community, and flowspec redirect ipv6

bgpd: support for bgp ipv6 ext community, and flowspec redirect ipv6

rfc 5701 is supported. it is possible to configure in bgp vpn, a list of
route target with ipv6 external communities to import. it is to be noted
that this ipv6 external community has been developed only for matching a
bgp flowspec update with same ipv6 ext commmunity.
adding to this, draft-ietf-idr-flow-spec-v6-09 is implemented regarding
the redirect ipv6 option.

Practically, under bgp vpn, under ipv6 unicast, it is possible to
configure : [no] rt6 redirect import <IPV6>:<AS> values.

An incoming bgp update with fs ipv6 and that option matching a bgp vrf,
will be imported in that bgp vrf.

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