stdio-bridge: Add --quiet option

stdio-bridge: Add --quiet option

When we use stdio-bridge via sd-bus to connect to a bus of a different
user, container or host, stdio-bridge should not log at error level but
at debug level as it's invoked by the sd-bus library and sd-bus should
generally not log above debug level.

We can't actually use the --quiet option yet as that would break connecting
to hosts running older versions of systemd but let's already add the option
now in preparation for a brighter future.