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
This should really be some form of g_try ( either _new0 or _malloc0 ) per the style guide.  I checked and there are roughly ~9 instances in the whole code base where we use g_malloc() and 8/9 are located in code that we've written ( vs. code ofono code  ).