Add --quiet flag to suppress log spam

[KogasaPls]

This pull request adds a --quiet flag to suppress log spam, fixing #22 (which has been noted by many others for the last decade). Specifically, it suppresses three types of messages:

"Supervising _ threads of _ processes of _ users." "Successfully made thread _ of process _ owned by _ high priority at nice level _." "Successfully demoted thread _ of process _."

These messages (especially the first two) are responsible for an annoyingly large amount of journal spam. It is convenient to have the ability to turn them off, but I have made no changes to the default behavior.

[jacobmcnamee]

rtkit is significantly more verbose than other services even when it has nothing useful to share. I have pages and pages of this:

rtkit-daemon[4850]: Supervising 0 threads of 0 processes of 0 users.

This PR has been open for almost a year. Is there anything preventing its merge?

[jacobmcnamee]

Alternative solution for systemd users: https://github.com/heftig/rtkit/issues/22#issuecomment-1321085246

[TriMoon]

Thanks, i'm also using the following, which eliminates the debug-level messages that generate the "Supervising" ones :wink:

IMHO this setting, in the drop-in, should be shipped by default, which would eliminate the need for this PR :vulcan_salute: (See: https://github.com/heftig/rtkit/pull/31)

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# /etc/systemd/system/rtkit-daemon.service.d/override.conf
[Service]
LogLevelMax=info