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_bssap.c b/openbsc/src/osmo-bsc/osmo_bsc_bssap.c
index a60940d..f38c97f 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_bssap.c
@@ -185,7 +185,7 @@
 	if (conn->conn) {
 		LOGP(DMSC, LOGL_INFO, "Releasing all transactions on %p\n", conn);
 		gsm0808_clear(conn->conn);
-		subscr_con_free(conn->conn);
+		bsc_subscr_con_free(conn->conn);
 		conn->conn = NULL;
 	}