Changes

gsupclient: Introduce gsup_client_mux

GSUP supports different message classes (e.g. SMS, USSD, Subscriber management),
within a daemon, the messages are usually handled in different parts of it.
The gsup client mux allow to place callbacks for the different message class.

This code original comes from the osmo-msc code and is here
re-licensed under the GPLv2.

Change-Id: I916c1bb6cfc48f92fc5db2b5c914eb1b4e25fa7f