carp: refactor packet tagging for ether_output()

• Separate HMAC preparation (CARP specific) from tagging.
• In unicast mode (CARP specific) don't put tag at all.
• Don't put pointer to software context into the tag. Putting just vhid, an integer value, is a safer design.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D45038