rename gsm_subscriber_connection to ran_conn

In preparation for inter-BSC and inter-MSC handover, we need to separate the
subscriber management logic from the actual RAN connections. What better time
to finally rename gsm_subscriber_connection.

* Name choice:

In 2G, this is a connection to the BSS, but even though 3GPP TS commonly talk
of "BSS-A" and "BSS-B" when explaining handover, it's not good to call it
"bss_conn": in 3G a BSS is called RNS, IIUC.

The overall term for 2G (GERAN) and 3G (UTRAN) is RAN: Radio Access Network.

* Rationale:

A subscriber in the MSC so far has only one RAN connection, but e.g. for
inter-BSC handover, a second one needs to be created to handover to. Most of
the items in the former gsm_subscriber_connection are actually related to the
RAN, with only a few MM and RTP related items. So, as a first step, just rename
it to ran_conn, to cosmetically prepare for moving the not strictly RAN related
items away later.

Also:

- Rename some functions from msc_subscr_conn_* to ran_conn_*
- Rename "Subscr_Conn" FSM instance name to "RAN_conn"
- Rename SUBSCR_CONN_* to RAN_CONN_*

Change-Id: Ic595f7a558d3553c067f77dc67543ab59659707a
diff --git a/include/osmocom/msc/signal.h b/include/osmocom/msc/signal.h
index f630b27..5126976 100644
--- a/include/osmocom/msc/signal.h
+++ b/include/osmocom/msc/signal.h
@@ -83,11 +83,11 @@
 	int paging_result;
 
 	/* NULL in case the paging didn't work */
-	struct gsm_subscriber_connection *conn;
+	struct ran_conn *conn;
 };
 
 struct scall_signal_data {
-	struct gsm_subscriber_connection *conn;
+	struct ran_conn *conn;
 	void *data;
 };
 struct sms_signal_data {