libbsc and libmsc have conflicting definitions of gsm_subscriber_connection
and do no longer belong together anyway.
Create libxsc, meaning 'lib[bm]sc', to hold all code used by both libmsc
and libbsc, and make sure gsm_subscriber_connection isn't used there.
In various binaries and tests, do not link libbsc and libmsc.
(Note: this commit was reshaped out of a large wip chunk, it may not
compile properly without the subsequent commits)