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
The naming convention for mtk_data isn't consistent ( sometimes it's called 'ril' and in this case it's called 'mtk' ).   It's probably wiser in this case to keep the convention and use 'ril' here, as the rest of this plugin uses that.  I actually thought there was a bug in your changes because I could find where *ril->modem* was initialized.