Allow the firewall rules to be established by a shell script instead

of forcing them to be an 'ipfw' rules file.  This allows one to
determine interface addresses dynamically, etc.  The rule is if the
file referenced by ${firewall_type} is executable, it is sourced, but
if it is just readable, it is used as input to 'ipfw' like before.

etc/rc.firewall

@@ -228,7 +228,9 @@ case ${firewall_type} in
 [Uu][Nn][Kk][Nn][Oo][Ww][Nn])
 	;;
 *)
-	if [ -r "${firewall_type}" ]; then
+	if [ -x "${firewall_type}" ]; then
+		. ${firewall_type}
+	elif [ -r "${firewall_type}" ]; then
 		${fwcmd} ${firewall_flags} ${firewall_type}
 	fi
 	;;