Greeter should honor "debug" kernel command line parameter

To debug greeter issues, it would be very useful to have debug logs of the greeter. To do that, one currently need to set a root password, go to a text console, change a file and restart the greeter. Not only this is a comlplicated process, but it also make it impossible to debug issues that only happen at the 1st start of the greeter. Let’s make the greeter set logging level to debug when it sees “debug” on the kernel command line.

Feature Branch: feature/12373-honor-debug-kernel-cmdline

Parent Task: #8230 (closed)

Related issues

Related to #12399 (closed)
Related to #12396
Related to #12395

Original created by @alant on 12373 (Redmine)

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