SXK80: Allow Specific Client/s across VLAN

BruceGuo it's actually pretty amazing. From my tests, I can keep network isolation on for the source vlan and access still works. Further, the gateway is doing single direction resolution. In the past, I've tried to use the avahi gateway but ran into issues with it essentially echoing back to hosts their own name which causes Apple produces to change their name to "some device (123)" and on macOS display a dialogue about the name being taken. This resolves that.

So, it looks like the gateway is discovering devices according to the Shared Service Type, creates iptables prererouting/redirect rules, and then relaying the broadcast? This is far better than what was done before and I'm impressed.

The only thing I'd ask to change is that the dropdown for services is someone restrictive. AFP is dying, AirPlay and Chromecast are good as are scanners and printing, but there should be a middle ground before hitting "All Services". For example, HomeKit is missing which is probably the second most used behind AirPlay/Chromecast. I would suggest giving people a custom option to add something like "_scanner._tcp", "_sonos._tcp", or whatever else.

Anyways, tomorrow I'll finally move my IoT devices off to their own VLAN and begin long term testing.

Once again, great work and thanks BruceGuo