Add new ICMPv6 counters for Anti-DoS limits.

Add four new counters for ND6 related Anti-DoS measures.
We split these out into a separate upfront commit so that we only
change the struct size one time. Implementations using them will
follow.

PR: 157410
Reviewed by: melifaro
MFC after: 2 weeks
X-MFC: cannot really MFC this without breaking netstat
Sponsored by: Netflix (initially)
Differential Revision: https://reviews.freebsd.org/D22711