Add initial VRRPv3 support (RFC 5798)

This commit will add a VRRP configuration option:
  version <VERSION>
where version can be "2" or "3".
Using version 2, which is the default, will send packets in VRRPv2 format
(RFC 3768), i.e. unchanged behavior from previously. By using version 3,
the packets will be send in VRRPv3 format instead (RFC 5798).
This commit does not make changes to the advertisement interval, i.e.
the advert_int configuration is still configured in seconds.

Signed-off-by: Jonas Johansson <jonasj76@gmail.com>