Draft: brltty start improvements

upstream udev and systemd support is used to start the brltty daemon when the display is connected. if the user wants to configure brltty himself, he can use the brltty@.service to start brltty with a specific config.

In addition, brltty now runs as a separate user, which improves system security.

p.s. I'm just starting to learn the process of creating packages for debian so I may well have missed something.