bgpd: support for redirect ipv6 simpson method

bgpd: support for redirect ipv6 simpson method

this commit supports [0] where ipv6 address is encoded in nexthop
attribute of nlri, and not in bgp redirect ip extended community. the
community contains only duplicate information or not.
Adding to this, because an action or a rule needs to apply to either
ipv4 or ipv6 flow, modify some internal structures so as to be aware of
which flow needs to be filtered. This work is needed when an ipv6
flowspec rule without ip addresses is mentioned, we need to know which
afi is served. Also, this work will be useful when doing redirect VRF.

[0] draft-simpson-idr-flowspec-redirect-02.txt

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