main: Allow websockets to use fd and unix sockets

The docs for the `--unix-socket` and `--external-listener-fd` options sound like they should work with the `--websocket` option. This change implements that. By allowing wayvnc to accept an arbitrary FD to listen on, we can use systemd's socket activation functionality to start wayvnc on demand. I am adding this as I want to put wayvnc behind nginx, and only start wayvnc when a webclient is trying to connect to the backend. -- `I have read and understood CONTRIBUTING.md` NOTE: I was not exactly clear on how you prefer lines to be wrapped, I believe I have followed style guide, but am not certain.