luci-lua-runtime: Update sys.net.conntrack() to use to conntrack tool

luci-lua-runtime: Update sys.net.conntrack() to use to conntrack tool

Fall through to /usr/sbin/conntrack tool if /proc/net/nf_conntrack is
not available.

/proc/net/nf_conntrack has been obsoleted in recent kernels
(https://cateee.net/lkddb/web-lkddb/NF_CONNTRACK_PROCFS.html). This
change enables sys.net.conntrack() to return conntrack information via
the /usr/sbin/conntrack tool (if installed) instead.

Enables population of the /luci/admin/status/realtime/connections list.

Signed-off-by: James Fox <[email protected]>
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Signed-off-by: Paul Donald <[email protected]>