Support 3g switch

This provides support on the ofono side for

https://bugs.launchpad.net/ubuntu/+source/ofono/+bug/1373388

set-3g-slot \<modem_path\>

modifies the slot with 3G capabilities. Note that once 3G has been set for a different slot to the one that had the capability, a full modem reset is needed for both modems:

offline-modem /ril_0
offline-modem /ril_1
online-modem /ril_0
online-modem /ril_1
"Only one of the modems can *have* this..."