Also provides systemd units.

It's normally safe to use --table=0 because most nodes use SLAAC and by default,
any existing default route is deleted.
It's better to abort if someone who explicitely wants to use a separate table
whereas the kernel is limited.

'/usr/bin/env python' also does not set useful process name for start-stop-daemon

This fixes a regression in commit b10674f3
("re6stnet: new client-only and routing-only mode").