Changes

s1ap_proxy: rework handle_ie/2 into handle_ie/3

This patch prepares for a follow-up change adding E-RAB MODIFY REQ/RSP.

Instead of passing the whole record #'ProtocolIE-Field' to handle_ie(),
pass the IEI ('id' field) and IE content ('value' field) as separate
arguments.  This split-up allows to pass the full IEI path to
handle_ie() in a follow-up change, and also allows to expand some
fields of the IE content right in the function header.

Change-Id: Ic71c8c07739b0e354ca9c9b03c000a366d8c2c43