nft-qos: support mac address based speed limit

nft-qos: support mac address based speed limit

This patch makes it possible to configure and limit per-client internet
speed based on MAC address and it can work with SQM.
This feature is what OpenWRT currently lacks. This patch is largely based
on static.sh and the configuration file is similar to original nft-qos.

New configuration options and examples are listed below

config default 'default'
    option limit_mac_enable '1'
config client
	option drunit 'kbytes'
	option urunit 'kbytes'
	option hostname 'tv-box'
	option macaddr 'AB:CD:EF:01:23:45'
	option drate '1000'
	option urate '50'
config client
	option drunit 'kbytes'
	option urunit 'kbytes'
	option hostname 'my-pc'
	option macaddr 'AB:CD:EF:01:23:46'
	option drate '3000'
	option urate '2000'

limit_mac_enable - enable rate limit based on MAC address
drunit - download rate unit
urunit - upload rate unit
macaddr - client MAC address
drate - download rate
urate - upload rate

Signed-off-by: Tong Zhang <ztong0001@gmail.com>