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
Differential Revision: https://reviews.freebsd.org/D41981