split subscr_con_allocate()/_free() in bsc_ and msc_

Rename current subscr_con_allocate() and subscr_con_free to bsc_*,
and add two separate msc_subscr_con_allocate() and _free().
The msc_subscr_con_free() ignores all lchan members.

In libbsc use bsc_*, in libmsc use msc_*.

Change-Id: I3cf7c7cafdf4672ec7b26058bba8a77159855257
Future: there will be distinct subscr conns for libbsc and libmsc.
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_sccp.c b/openbsc/src/osmo-bsc/osmo_bsc_sccp.c
index 2fafed6..a571438 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_sccp.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_sccp.c
@@ -84,7 +84,7 @@
 			LOGP(DMSC, LOGL_ERROR,
 				"ERROR: The lchan is still associated.\n");
 			gsm0808_clear(con_data->conn);
-			subscr_con_free(con_data->conn);
+			bsc_subscr_con_free(con_data->conn);
 			con_data->conn = NULL;
 		}
 
@@ -107,7 +107,7 @@
 {
 	if (data->conn) {
 		gsm0808_clear(data->conn);
-		subscr_con_free(data->conn);
+		bsc_subscr_con_free(data->conn);
 		data->conn = NULL;
 	}