ipfw.8: Adjust section for loader tunables

Move the descriptions of loader tunables from section 'SYSCTL VARIABLES'
to section 'LOADER TUNABLES'.

See also 49197c391b3d (ipfw: Add sysctl flag CTLFLAG_TUN to loader tunables).

MFC after: 2 days
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D41981

(cherry picked from commit 12349f38898f231ca803dcf526bac88cb1b5cd2b)
(cherry picked from commit bb6f9a95402a6c3ab8167481b81465f8ad5016fc)