Introduce iu_rnc FSM

This FSM is similar to the already existing ran_peer_fsm in osmo-msc,
which already had better logic around SCCP and RANAP state handling.
Similarly, osmo-sgsn's struct ranap_iu_rnc maps to osmo-msc's struct ran_peer.

With this FSM we can currently track the RANAP link state towards a given
RNC peer:
* Reject (RANAP Error Indication) all UE-related messages until a RANAP
  RESET from RNC is received first.
* Tear down all subsriber connections whenever the RANAP peer sends us a
  RESET message.
* Tear down all subscriber connections whenever the SCCP link towards
  RNC becomes unavailable.
* Send a RESET towards RNC peer once the SCCP link towrdards it becomes
  available again.

This commit only implements so far the Rx path of the FSM, ie. when
receiving events/messages from a peer over SCCP and pushing them locally
up the stack (RANAP). The Tx side will be implemented in a follow-up
commit, which will allow discarding messages if the lower layers towards
a given RNC are known to be down.

Change-Id: I18b7803500163e78ff6a684095194174b0fb6ee1
---
M include/osmocom/sgsn/Makefile.am
M include/osmocom/sgsn/gprs_ranap.h
M include/osmocom/sgsn/iu_rnc.h
A include/osmocom/sgsn/iu_rnc_fsm.h
M include/osmocom/sgsn/sccp.h
M include/osmocom/sgsn/sgsn.h
M src/sgsn/Makefile.am
M src/sgsn/gprs_ranap.c
M src/sgsn/iu_rnc.c
A src/sgsn/iu_rnc_fsm.c
M src/sgsn/sccp.c
M src/sgsn/sgsn.c
M src/sgsn/sgsn_vty.c
M tests/gprs_routing_area/Makefile.am
M tests/osmo-sgsn_test-nodes.vty
M tests/sgsn/Makefile.am
16 files changed, 591 insertions(+), 95 deletions(-)