Debian: Made handling of features-enabled respecting disabled features.

The default features now only get enabled on fresh installs. This has been
changed for icinga2-common and icinga2-classicui.

Fixes #5102
Debian: Fixed config check and changed DAEMON_ARGS in init script.

Check now actually works, command was incorrect and not properly blocking the
actual action on fail.

Also the config file is now in DAEMON_CONFIG variable.

Fixes #5086