quick and dirty add support for older Linux kernels

Older Linux kernels (RedHat/CentOS 5.x, others pre 2.6.32) publish the conntrack stats slightly differently.  This short patch quickly makes that work by attempting to open the old style (v4 only) conntrack before failing.  This could be improved to have a config switch, and to possibly look for the v6 conntrack stats as well on the older systems.