Groups are only used as firewall rules interfaces, so move them
into the "pf" "plugin" and fix the menu accordingly.

(plugins, interfaces) add ifgroup type, filter in interfaces_groups_edit.php. for https://github.com/opnsense/core/issues/1455

(gwlb.inc) more cleanups, $ifname == $gateway_item['friendlyiface'] looks kind of impossible here, $ifname is unique and not in $gateway_item yet...

It's a bit funny that the help text indicates we can use tag/tagged with
NAT rules as well, because this was never implemented although it really
works.

(firewall) empty means "in"... confusing ;) https://github.com/opnsense/core/blob/17.1.2/src/etc/inc/filter.inc#L2314-L2315

(firewall) add port/address parsing to FilterRule(), should be backwards compatible with current config settings

(filter) filter_generate_port, why all this trickery?? it's either a valid port range or it's not. not need to overreact.

It's better to replace this now, the new style code won't include this "magic".