Add qos_* iptables chain namespace for QoS

Add qos_* iptables chain namespace for QoS

Prefix the names of the iptables chains used for QoS with "qos_" to
allow for identification of QoS chains for removal across configuration
changes.  When clearing QoS iptables chains, only remove chains with the
namespace prefix in order to preserve user and/or other package chains
in the mangle table.

Signed-off-by: Kevin Locke <klocke@digitalenginesoftware.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28622 3c298f89-4303-0410-b956-a3cf2f4a3e73