Changes

iu_client: refactor LAC/RAC handling

The iu client ignored the PLMN which might ran into duplicate
RACs. Replace all lac/rac handling by the "new" osmo_routing_area_id
struct which has improved handling.
Further it allows to use the embedded lac when CS paging is only required.
The old gprs_ra_id is still used to contain api stability.
RAC with 0xff will be rejected (which should be invalid already, because
the SIM is using this value for an invalid entry).
Internally RAC = 0xff will be used to differentiate between LAI and RAI.

Change-Id: Ie38fa3751cfce1c981d8d0bed1b8ff891593a638