Changes

Use gsup_client_mux from libosmo-gsup-client

gsup_client_mux code was cloned a couple months ago to
libosmo-gsup-client, use it in osmo-msc and drop its own copy.

The code was not dropped at the time because it required tweaking the
unit tests, because they were wrapping some underlaying functions which
cannot be wrapped anymore since the code using them is now inside the
separate already-linked libosmo-gsup-client.so.
Solve it by actually wrapping one layer up in the gsup_client_mux itself
since it now something we mantain here anymore.

Change-Id: I34d02ae219f3ec8daf47942c46f52b21eab21242