carp: Set DSCP value CS7

Update carp to set DSCP value CS7(Network Traffic) in the flowlabel field of
packets by default. Currently carp only sets TOS_LOWDELAY in IPv4 which was
deprecated in 1998. This also implements sysctl that can revert carp back to
it's old behavior if desired.

This will allow implementation of QOS on modern network devices to make sure
carp packets aren't dropped during interface contention.

Submitted by: Nick Wolff <darkfiberiru AT gmail.com>
Reviewed by: kp, mav (earlier version)
Differential Revision: https://reviews.freebsd.org/D14536