add systemd unit file

Problem: we currently start brewcop from the brewcop user's `.bash_login`, but that is irritating to control when developing.

Just add a systemd unit file so we can start and stop it with systemd.  We can map it to `/dev/tty1` or one of the other virtual consoles using

https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#TTYPath=