During system start up pppoe devices seem to receice ifup events before
the interface actually exists. This commit makes sqm's run.sh script
test whether the sys files for an interface exist before actually trying
to start an SQM instance on an interface. This seems to nicely avoid
starting on an not fully established pppoe interface and avoids a number
of error messages during startup.

In addition, debug logging is disabled.

Signed-off-by: Sebastian Moeller <moeller0@gmx.de>