scheduler/ipp.c: Allocate device_uri via cupsdSetString()

scheduler/ipp.c: Allocate device_uri via cupsdSetString()

If a driverless printer has .local in its URI, we resolve the
URI and save the resolved one as new device URI. The problem was that
a local pointer was assigned to the structure which is passed to the
function as parameter, so the pointer became invalid once the execution
left the create_local_bg_thread() function.

We need to allocate the device URI via cupsdSetString() - the string is
then freed when the printer is deleted or cupsd shuts down.

Fixes #419.