Changes

Include SM-RP-UI in outgoing GSUP message when presesnt

An SMS deliver report may be returned from the UE for any MT SMS.  A
common use is for remote communication with the SIM (UICC) whereby a
command requires Proof of Receipt in an SMS deliver report (see 3GPP
31.115)

SMS-DELIVER-REPORT TPDU is defined in 3GPP 23.040 s9.2.2.1a and
carried as a RP-User-Data element within an RP-ERROR or RP-ACK PDU

In 3GPP 24.011, the definition of RP-ACK (s7.3.3) and RP-ERROR
(s7.3.4) shows RP-User-Data encoded as an optional TLV.

In MAP (3GPP 29.002) the MSC transfers the user data in either
SM-RP-UI parameter (s7.6.8.4) or SM-DeliveryFailureCause
diagnosticInfo (s7.6.1h)

With GSUP the osmo_gsup_message struct member sm_rp_ui is used to
carry the user data in both cases.

Change-Id: I90a607900ac7c86b200c79b7e8c460d4cbb3c3ce