Predefined channles: Fix handling of legacy configuration options

Predefined channles: Fix handling of legacy configuration options

Fix the handling of legacy "Key" and "MaxUsers" [Channel] settings:

- Activate them before evaluating the "Modes" parameter, to allow the
  latter to override those legacy options.
- Enforce setting the respective +k/+l mode(s) to support the legacy
  "Mode = kl" notation, which was valid but is an invalid MODE string:
  key and limit are missing! So set them manually when "k" or "l" are
  detected in the first MODE parameter.
- Sort modes +kl alphabetically, adjust test suite accordingly.