Rec: systemd-journal structured logging followup

[omoerbeek]

Short description

As @mnordhoff noted, the new systemd-journal backend isn't nice for human consumption. So switch back to the row of k=v pairs output by default and introduce a setting to use the systemd-journal backend. This setting also allows for other backends to be used (in the future).

Also: respect loglevel. The traditional output was filtered, but for structured logging, that was not done. While there, shortcut the traditional way as well, to avoid unnecessary churn.

Should fix #11705 and #11706

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[fredmorcos]

Minor stuff, nothing blocking though.