nixos/brltty: use upstream units
nixos/brltty: use upstream units

Upstream has been providing a very thoroughly designed set of systemd units,
udev and polkit rules. With these the brltty daemon is activated
asynchronously via udev, runs as a dedicated user with runtime and state
directories set up using systemd-tmpfiles.

This is much better than the current unit, which runs a single instance
as root and pulls in systemd-udev-settle to wait for the hardware.

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