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

carp: allow commands to use interface name rather than index

Get/set commands can now choose to provide the interface name rather
than the interface index. This allows userspace to avoid a call to
if_nametoindex().

Suggested by: melifaro
Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39359

carp: support unicast

Allow users to configure the address to send carp messages to. This
allows carp to be used in unicast mode, which is useful in certain
virtual configurations (e.g. AWS, VMWare ESXi, ...)

Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D38940

carp: add netlink interface

Allow carp configuration information to be supplied and retrieved via
netlink.

Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39048