vrrp: add support to IPv6 mcast src address specification

For some reason (well... which one ?), previous code didnt support specification
of multicast source address in IPv6 mode. If you are using 'native_ipv6' and want
to specify IPv6 mcast source ip address then you can use 'mcast_src_ip' keyword
with IPv6 address.
vrrp: Add support to IPv6 src_address discrimination in master rx state

Previous code didnt support IPv6 address discrimination while in MASTER state
receiving same prio advert. This patch extend previous code to support IP
address comparison agnostic.
vrrp: Add missing IP address comparison while receiving same prio advert in MASTER

Pevious code updated link and sent advert in MASTER mode if received advert was
lower prio than local instance prio. Extended the code to handle the same case
where incoming advert is the same prio but ip_src is lower than local instance
ip_src. This patch just speed up the election by automatically update remote
cache (v4 or v6).
vrrp: revert previous patch
vrrp: re-apply previously reverted
vrrp: after code review by a freind :D