Updated AT&T APNs and APN usages

The AT&T APNs were ancient, but to update them to the current AT&T APNs required specifying multiple usages, which was not possible with the current accepted syntax. I added the possibility of using a chain of usages in one usage i.e. usage="<usage1>-<usage2>-...". It's not a perfect system, but it will work until someone needs a larger number of combinations of usages, as the ones I use are hard-coded.