Add systemd service file

[wrobell]

The service file loads the kernel module and starts VNC server.

[pl-semiotics]

Thanks for putting this together! I do have a couple of minor concerns re: merging it, though:

• The unit file makes some assumptions about the installed locations of the module/server binaries that aren't documented anywhere
• It looks like this is set up to allow automatically starting the server. I'm a little hesitant to make that a default thing to do, since the rM appears to have no firewall whatsoever, and the server binds to all network interfaces (so running it with the rM on an untrusted WiFi network, without iptables rules, would be not ideal).

[wrobell]

1. Would you like me to propose some installation instructions in the readme file?
2. Someone has to run systemd enable rm-vnc-server to make it start after every reMarkable reboot.