netinet6: Add sysctl flag CTLFLAG_TUN to loader tunables

The following sysctl variables are actually loader tunables. Add sysctl
flag CTLFLAG_TUN to them so that sysctl -T will report them correctly.

1. net.inet6.ip6.auto_linklocal
2. net.inet6.ip6.accept_rtadv
3. net.inet6.ip6.no_radr

No functional change intended.

Reviewed by: glebius
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41928

(cherry picked from commit 03dac3e37993801dab4418087bfedacce0526e66)
(cherry picked from commit da2b630c12ec074673dfc646b2055a31b0d8d6d9)