Need more checks on global parameters.
Reference #576 (comment 61000) second problem: invalid values for global parameters should be rejected even they are not used:
- for individual parameters, e.g. next-server sets to something not parse-able as an IPv4 address (nor empty)
- between parameters, e.g. 0 <= t1_percent <= 1 and t1_percent < t2_percent
The ideas are:
- avoid cases where the config backend is used to create a configuration which can't be saved and reloaded (get then set or write then reload fails)
- avoid cases where a global parameter with an invalid value is added/accepted and the invalid will raise an error a long time ago when an update will first use it (e.g. configuration built incrementally starting by global parameters, IMHO could be a popular way to proceed).