libmsc/gsm_09_11.c: implement network-initiated sessions

This change introduces a possibility to establish network-initiated
SS/USSD transactions with a subscriber in either IDLE, or DEDICATED
state. In the first case, a new transaction is established using
Paging procedure. If a subscriber already has an active connection,
a separate new transaction is established.

TTCN-3 test case: I073893c6e11be27e9e36f98f11c1491d0c173985
Change-Id: Ief14f8914ef013bd6efd7be842f81fbf053f02e2
diff --git a/include/osmocom/msc/transaction.h b/include/osmocom/msc/transaction.h
index 63b1cc5..4ffb468 100644
--- a/include/osmocom/msc/transaction.h
+++ b/include/osmocom/msc/transaction.h
@@ -77,6 +77,13 @@
 
 			struct gsm_sms *sms;
 		} sms;
+		struct {
+			/**
+			 * Stores a GSM 04.80 message to be sent to
+			 * a subscriber after successful Paging Response
+			 */
+			struct msgb *msg;
+		} ss;
 	};
 
 	struct {