Changes

Adopt iu_client from osmo-iuh

iu_client was original written to have an abstraction
for Iu. However the MSC stopped using iu_client a long
time ago because the abstraption didn't fit well.
There was never time to rewrite SGSN to direct talk RANAP
using the primitives.
Adopt iu_client for now, but with the plan to rewrite
it in a similar way as osmo-msc is doing IuCS.

Adopted from osmo-iuh:
1631c8ac5a2f ("iu_client: fix linter issues")
Related: OS#5487

Change-Id: I9c0d9667ddcb57b52eae34da942acfc1688c1418