systemd: restart l2tpns on failure

When a l2tpns cluster gets out of a "split brain" situation, l2tpns will consider that it is not normal to having multiple master, and some members of the cluster will voluntarily kill themselves.

While this is probably the best thing to do for l2tpns in this situation, continued operation is important. Let's tell systemd to restart l2tpns in such cases.

Looking at the code, there are several other places where l2tpns might exit in case of grave failure, and this change will also cover these cases.