bugfix for firewall module : 1.6 released

The code was generating a bad configuration file for linux kernel 4.x, and the module was sending alarm messages such :
WARNING on module firewall main::read_pipe_stream(/etc/webmin/sysstats/modules/firewall/../../sysstats-gen-lib.pl 606) : can not open piped command /bin/ -L -n -v -x 2>&1 | : Permission denied
Use of uninitialized value $pipe in at ./firewall.pl line 93.