carp: support VRRPv3

Allow carp(4) to use the VRRPv3 protocol (RFC 5798). We can distinguish carp and
VRRP based on the protocol version number (carp is 2, VRRPv3 is 3), and support
both from the carp(4) code.

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D44774