vrrp: Tweak IPv6 VIP install by disabling DAD algo and setting deprecated flag

Lot of discussions have been made around those 2 topics. First idea and initial
patch where provided by Leo Baltus. This patch fix the use case where VRRP VIPv6
are used in conjonction of IPVS healthchecking. If deprecated flag is not set
(which is the default linux behaviour), then VRRP VIP can be used as source address
of healthcheking packet. Since this VIP address is also present, in most use-cases,
on realserver directly so return packets never reach the healthchecker and hence no
realserver was injected in IPVS table.

At the same time, I decided to merge Tore Anderson  suggestion of disabling
Duplicate Address Detection algorithm. Tore's arguments are nice ! Thanks Tore :

Using the nodad flag has the following benefits:

1) The address becomes immediately usable after they're configured.
2) In the case of a temporary layer-2 / split-brain problem we can avoid
that the active VIP transitions into the dadfailed phase and stays there
forever - leaving us without service. HA/VRRP setups have their own
"DAD"-like functionality, so it's not really needed from the IPv6 stack.

Acknowledgements to Mark Schouten and Frank Baalbergen for pushing me by
testing this features !
Merge branch 'master' of github.com:acassen/keepalived