The idea seems to be that when usb-moded is executed in "normal manner" it
logs warnings / more server issues to syslog and possible output from
subprocesses executed by usb-moded gets redirected to /dev/null to avoid
journal spamming. However, now the redirection happens also when explicitly
requesting logging to stderr.

Redirect standard outputs only when neither debug logging nor logging to
stderr is requested.
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>