Changes

library/gsup: distinguish GSUP_MT_FORWARD_SM_ERR_{MS,NET}

The idea is to have two variants of the MT_FORWARD_SM_ERROR:

* _MS: originated by the MS/UE (via RP-ERROR),
* _NET: originated by the network itself.

The network-originated kind will be used to indicate unsuccessful
delivery of a MT SMS in absence of response from the MS/UE.

Change-Id: I58e3566731d232b731b2c0be104d8a1098bbf21b
Related: SYS#7323