session: Redirect output of cosmic-comp

[Drakulix]

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[mmstick]

@Drakulix Is this good to merge?

[Drakulix]

If we want to merge this (which I am not sure), then only in combination with pop-os/cosmic-comp#1159 otherwise we spam journald with two copies of the logs.

The intent of this PR was to capture log outputs of our c-library dependencies (like seatd) somewhere, since we can't easily redirect them.

Another option would be to completely disable journald logging under cosmic-session in cosmic-comp, bringing it more in-line with the rest of the components. However that looses all the key-value data, which is very helpful for debugging.

A better fix for this whole mess would be to figure out how to parse and keep/forward that data in cosmic-session and then drop the journald code from cosmic-comp. But that seems to be more a long-term goal.