Teach osmo-bsc to handle empty N-Connect. So far we were always
expecting user data in an SCCP N-Connect from an MSC. However, it is
perfectly valid for an initial BSSMAP request to follow later.
This is relevant for:
- Handover Request (incoming inter-BSC handover)
- Perform Location Request (query physical location of the MS)
Add state WAIT_INITIAL_USER_DATA with new timeout net X25. Always enter
this state so that we don't have two separate code paths for handling
initial user data.
Related: SYS#5864
Change-Id: I535c791fa01e99a2226392eb05f676ba6c3cc16e