allow maxlen for domains to be specified

[luto]

trafficstars

Domains should have a maxlen and maxcomponents parameters, specifying the maximum length or the maximum number of components a domain may have. They should default to the values currently implemented and suggested by the spec. The new parameters must only allow lower values than the current ones.

[brutus]

The maxlen part is implemented by https://github.com/Uberspace/paternoster/pull/40 (see also https://github.com/Uberspace/paternoster/issues/39). But not asserting that no values higher than the hardcoded defaults are used and also not the maxcomponents argument.