This reduces the probability to lose all connections at the same time when a
gateway becomes out-of-order.

This bug could even freeze re6st completely when trying to rejoin the network.

Non-numerical address should be rare and this could even be counter-productive:
such remotes are more likely to host other services and forcing routes may
break already opened non-re6st connections.

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.