fix #607: DUPLEX_UNKNOWN is not defined on old RedHat versions
fix #607: DUPLEX_UNKNOWN is not defined on old RedHat versions

qbit/psutil,cloudbase/psutil,Q-Leap-Networks/psutil,qbit/psutil,mrjefftang/psutil,mrjefftang/psutil,jorik041/psutil,jorik041/psutil,jamesblunt/psutil,cloudbase/psutil,packages/psutil,0-wiz-0/psutil,cloudbase/psutil,jamesblunt/psutil,msarahan/psutil,giampaolo/psutil,giampaolo/psutil,qbit/psutil,Q-Leap-Networks/psutil,packages/psutil,mrjefftang/psutil,msarahan/psutil,jorik041/psutil,0-wiz-0/psutil,jamesblunt/psutil,packages/psutil,msarahan/psutil