Draft: logger: Don't output to terminal if told to use Journal exclusively

• !719 (merged)

• logger: Don't output to terminal if told to use Journal exclusively

  This makes the behaviour of:

    SRT_LOG=journal srt-logger -d ~/tmp -- s-r-something

  consistent with:

    SRT_LOG=journal s-r-something

  when run from a terminal, and similar for SRT_LOG_TO_JOURNAL=1.

  However, it also causes

    SRT_LOG=journal srt-logger -d ~/tmp -- some-unrelated-game

  to supress output to the terminal, which might be unexpected.

  steamrt/tasks#466

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/cc @refi64 @denittis

I'm not convinced we actually want this change... but if we do, this is how it could be implemented.