Define masks for iptables marking in functions.sh

Openwrt's qos-scripts uses masks in iptables to only write its markings for
filtering into 8 bits so multi-wan and other packets can use other bits of the 32bit
mark. Since qos-scripts and SQM are mutually eclusive we just reuse the same mask.
Teach simple.qos ond derivatives to use masks for iptables markings

This change hopefully allows SQM to coexists with multi-wan...