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_api.c b/openbsc/src/osmo-bsc/osmo_bsc_api.c
index 7a3ef70..49e5796 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_api.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_api.c
@@ -330,7 +330,7 @@
 	_conn->sccp_con = NULL;
 	if (complete_layer3(_conn, msg, msc) != BSC_API_CONN_POL_ACCEPT) {
 		gsm0808_clear(_conn);
-		subscr_con_free(_conn);
+		bsc_subscr_con_free(_conn);
 		return 1;
 	}