Allow starting stock xochitl by accessing a different port

[Eeems]

In the situation where someone has soft-bricked due to the UI no longer being available (rm2fb not supporting their OS, and failing to fallback for example), it would be useful to allow a user to trigger stock xochitl by port knocking. This would start a systemd service that explicitly starts the stock xochitl without any of the toltec changes. It would also need to stop any potentially running launchers, as well as rm2fb. The code would have to be written in an extremely fault-tolerant way, and probably as a shell script to ensure it will continue to work on newer OS versions.

Systemd supports activating a service from a socket, including ports: https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html

It would probably make sense to limit this to only over the USB interface.