luci-mod-status: amend luci-bwc.c to use conntrack tool

luci-mod-status: amend luci-bwc.c to use conntrack tool

Fall through to /usr/sbin/conntrack tool if /proc/net/nf_conntrack or
ip_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 luci-bwc to collect conntrack information via the
/usr/sbin/conntrack tool (if installed) instead.

Populates the /luci/admin/status/realtime/connections chart.

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