raspberrypi-gpioshutdown kernel module:

This port installs a startup script in your ${LOCALBASE} etc/rc.d directory
called 'gpioshutdown'.

Once installed, either load the kernel module at boot time (using
/boot/loader.conf) or by adding the following entry to the /etc/rc.conf file
(recommended) by running this command:

    sysrc gpioshutdown_enable="YES"

Or, you can simply load the module using the 'rc' system, by adding this
line to /etc/rc.conf instead:

    kld_list=gpioshutdown

If you have other modules in 'kld_list' just add 'gpioshutdown' to the end of
that list.

For more detailed information, type 'man 4 gpioshutdown'.