Fix SQM tear down on interfaces that are just deleted or changed in uci

Up until now SQM tried to use the uci config file /etc/config/sqm
to figure out on which interfaces it was running. Unfortunatelly,
the uci config file is a bad place to save this state, as the user
can change the interface for a given section and thereby wiping out
the information about the previous interface, whileSQM might still
be active on the old interface. To partly deal with this issue
SQM had started to create its own state file to mark interfaces
with active SQM instances. Instead of using the interfaces from the
config file just use these state fils to figure out which interfaces
might need to be stopped. And since all current qos scrpts start out
with tearing down any traffic shaping infrastructure on its main
interface and the associated IFB, we just stop all active SQM instances
before potentially starting new ones. This needs testing...