During a long time, we had both file and symbol names, actually
related to Supplementary Services, with the 'ussd' abbreviation.
This is not absolutely wrong, but isn't correct at the same time.
USSD is a kind of Supplementary Services, this is only a part
of them. There are also 'structured' Supplementary Services,
which can be call related or call independent.
The "Signalling interworking for supplementary services" is
defined by GSM TS 09.11, and this is exactly what MSC should
implement. Let's use the specification number for naming, as
we do e.g. in the GSM 04.11 (SMS) implementation.
Change-Id: Ic1eaceddb58132318e4e941be542da34b8ebefe1
5b860faf4e