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

(cherry picked from commit a03aff88a14448c3084a0384082ec996d7213897)