Changes

s1ap_proxy: handle_ies(): pass IEI path to handle_ie()

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

We need to be able to match the IEs unambiguously, because the same
IEI may be used in different contexts and thus have different meaning,
sometimes within the same PDU.  The E-RABItem IE is such an example:

* 9.1.3.5 E-RAB RELEASE COMMAND
** contains an E-RABToBeReleasedList IE
*** contains E-RABItem IEs

* 9.1.3.7 E-RAB RELEASE INDICATION
** contains an E-RABReleasedList IE
*** contains E-RABItem IEs

So far we handled this duality by passing the release kind via
the #proxy_state record ('rel_kind' field) and it worked.  But
soon in follow-up changes we'll have to deal with other PDUs:

* 9.1.3.4 E-RAB MODIFY RESPONSE
** contains an E-RABModifyListBearerModRes IE
** may contain an E-RABFailedToModifyList IE
*** contains E-RABItem IEs

* 9.1.3.9 E-RAB MODIFICATION CONFIRM
** may contain an E-RABFailedToModifyList IE
*** contains E-RABItem IEs
** may contain an E-RABToBeReleased IE
*** contains E-RABItem IEs

A universal approach allowing to match and handle IEs properly
depending on the context is to maintain the full IEI path
in handle_ies() and pass it to handle_ie().

Change-Id: I35528a91f7dbf321ad1a0811f1bbcdfb529d3530