Changes

Split most of iu_client.{c,h} into sccp.{c,h} and iu_rnc.{c,h}

* iu_rnc is easily spotted as an object class in the data domain which
  can be put into its own file, handling its specific logic and data.

* SCCP code can also be moved into own specific file, which eases
  self-containment of the layer logic and data.

* As a result, lots of RANAP Rx/Tx code in iu_client.{c,h} can also be
  moved to the already existing gprs_ranap.{c,h}.

All these changes improve a lot readibility of code and makes it a lot
easier to improve/extend the different layers/objects in future patches.

Change-Id: I4c792ae665720460b5a954b5c7cbfe5acbc37473