ipfw: 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.inet.ip.fw.enable
2. net.inet6.ip6.fw.enable
3. net.link.ether.ipfw

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 49197c391b3d49a33bcd08471bf64d533cd10f9d)
(cherry picked from commit f5f6da90597cb65ce55c4f17718a200097f6f78f)