[BRLTTY] Systemd service autostart

[Samuel Thibault]

Hello,

My previous mail was about udev-based systems.  For systemd-based
systems, the nicer thing would be to have brltty auto-started on USB
plug.  I haven't investigated about the details yet, but the idea would
be that systemd knows about the brltty service, which may be started
and stopped by handle, or enabled to start automatically at boot, or
auto-started on USB plug event.  As with the udev case, we'd need a
no-generic variant, which we could thus safely get installed on all
Linux systems in the world :)

Samuel