Added a "restartpause" option

This allows to set a pause between the restarts of the program.

Typically, when you restart a database server, this can take few minutes and the backoff delay won't be enough. With a restart pause delay of something like 30 seconds, this becomes a lot less likely. 
The alternative is to increase the number of start retries, but on system startup and/or when many programs fail at the same time (usually for the same reason), it creates an unnecessary workload on the host.

This changes the behaviors in two scenarios:
* Applied on startup failure additionally to the backoff delay
* Applied on bad exit status

Default value is 0, which doesn't change the existing behavior.

It is mostly a proposal at this stage. We could also allow to change the backoff increment (which is hardcoded to 1 at the current stage) or only the backoff base delay.