icmp6: bring rate limiting on a par with IPv4

Use counter_ratecheck() instead of racy and slow ppsratecheck. Use a
separate counter for every currently known type of ICMPv6. Provide logging
of ratelimit events. Provide jitter to counter open UDP port detection.

Reviewed by: tuexen, zlei
Differential Revision: https://reviews.freebsd.org/D44482