iuup: Rework API to support RFCI IDs != RFCI index

The initially merged IuUP API and implementation assumed that RFCI with
ID was always in the position of its ID inside the list of RFCIs. This
was the case for messages sent by ip.access nano3g as well as our own
osmocom implementation. However it was noticed that other nodes from
other vendors actually use other order, as allowed by the IuUP message
format.
Hence, we need to break the assumption and provide explicit ID
information in the list.

NOTICE: This commit breaks API and ABI compatibility with older versions
of libosmogsm, but not with any previous release of libosmocore since
the API is only available in master so far (it was added in
9fe1f9fb0b3197cdecaa55017d3afd7355e59c36).
Similary, it's only user (osmo-mgw) only uses the API in master, so
there's no API breakage with older releases.

Related: SYS#5969
Change-Id: Ib21cee2e30bf83dff4e167f79541796007af9845
---
M TODO-RELEASE
M include/osmocom/gsm/iuup.h
M src/gsm/iuup.c
M tests/iuup/iuup_test.c
4 files changed, 76 insertions(+), 48 deletions(-)