netfilter: xt_qtaguid: provide an iface_stat_all proc entry

There is a
  /proc/net/xt_qtaguid/iface/<iface>/{rx_bytes,rx_packets,tx_bytes,...}
but for better convenience and to avoid getting overly stale net/dev stats
we now have
  /proc/net/xt_qtaguid/iface_stat_all
which outputs lines of:
  iface_name active rx_bytes rx_packets tx_bytes tx_packets
    net_dev_rx_bytes net_dev_rx_packets net_dev_tx_bytes net_dev_tx_packets

Change-Id: I12cc10d2d123b86b56d4eb489b1d77b2ce72ebcf
Signed-off-by: JP Abgrall <jpa@google.com>