icmp6: move ICMPv6 related tunables to the files where they are used
Most of them can be declared as static after the move out of in6_proto.c.
Keeping sysctl(9) declarations with their text descriptions next to the
variable declaration create self-documenting code. There should be no
functional changes.
Differential Revision: https://reviews.freebsd.org/D44481
(cherry picked from commit 4f96be33fe7676c69c5abb476bb09bba0c63a3f4)