oFono/rilmodem does not handle SIM-stored SMS

Use cases:
 - Receive a class 2 SMS
 - Receive a message waiting indicator SMS
 - Actual result: Nothing happens

Some types of SMS are currently not received by oFono and by Ubuntu Touch. SMS are usually notified from the modem and the Android RIL API to oFono via RIL_UNSOL_RESPONSE_NEW_SMS. But certain types of messages are stored on the SIM card by the modem, namely messages with these properties in the Data Coding Scheme (see [1]):
  "Class 2 (U)SIM specific message"
  "Message Waiting Indication Group: Store Message"
These messages are notified to oFono via UNSOL_RESPONSE_NEW_SMS_ON_SIM, and they need to be separately read from the SIM card. That functionality is currently not implemented, oFono just prints one line to the log:
  Jul 3 05:06:40 ubuntu-phablet ofonod[1198]: gril/gril.c:handle_unsol_req() RIL Event: UNSOL_RESPONSE_NEW_SMS_ON_SIM

This is a legacy feature (from times when phones had no or very little storage for SMS), but some service providers still want SMS stored on the SIM.

For reference, such a functionality was recently added to the 'nemomobile' fork of oFono (used by Sailfish OS), see [2]. That implementation simply deletes the message from SIM after reading it, to avoid any "out of SIM memory" situations (not really 3GPP/GCF compliant but works).

[1] 3GPP TS 23.038 "Alphabets and language-specific information", section 4 "SMS Data Coding Scheme"
[2] https://github.com/nemomobile-packages/ofono/pull/235