Add support for debug cmdline option in Whisperback

As we’ve done with the Tails greeter (see #12373 (closed)), Whispeback should output its debug logs in the journal if the debug option is set in /proc/cmdline.

Feature Branch: feature/12399-support-for-debug-cmdline-option,https://gitlab.com/saschamarkus/whisperback/tree/feature/12399-support-for-debug-cmdline-option

Related issues

Related to #12373 (closed)
Related to #12396
Related to #12395

Original created by @bertagaz on 12399 (Redmine)

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