add public API: osmo_sccp_instance_next_conn_id()

In struct osmo_sccp_instance, we have a next_id, to find an unused SCCP
connection ID. This is used for inbound SCCP connections.

At the same time, in each of our SCCP client programs, we have a
separate mechanism to find a connection ID for outbound connections.

See for example bsc_sccp.c: bsc_sccp_inst_next_conn_id()

It makes much more sense for callers to use the same
osmo_sccp_instance->next_id counter:

- Currently the inbound and outbound counters go out of sync: For
  example, in osmo-bsc, if we have three MS doing a usual Complete Layer
  3, osmo-bsc's counter increments by three. If we then have one
  Handover Required coming back from the MSC, i.e. inbound SCCP
  connection, the sccp_inst->next_id is behind by three, and will
  iterate SCCP connections three times before finding an unused id.

- Each implementation has to take care to properly wrap the ID in its
  valid range, e.g. in osmo-bsc:

                test_id = (test_id + 1) & 0x00FFFFFF;
                if (OSMO_UNLIKELY(test_id == 0x00FFFFFF))
                        test_id = 0;

Add public API so that callers can benefit from the internal
osmo_sccp_instance->next_id and do not need to duplicate the code for
staying within the proper range.

Change-Id: If59d524fbe1088a59ae1b69908e2d4bf67113439
---
M include/osmocom/sigtran/sccp_sap.h
M src/sccp_scoc.c
2 files changed, 41 insertions(+), 1 deletion(-)