vrrp: disable TTL sanity check for unicast use-case

In order to protect against any packet injection, VRRP provides
sanity check over IP header TTL. This TTL MUST be equal to 255 and
means both sender and receiver are attached on the same ethernet
segment. Now with unicast extension this protection MUST be disabled
since VRRP adverts will mostly traverse different network segments.

!!! WARNING !!! When using VRRP in unicast use-case in order to protect
against any packet injection the best practice is to use IPSEC-AH auth
method otherwise you are exposed to potential attackers !