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/libmsc/osmo_msc.c b/openbsc/src/libmsc/osmo_msc.c
index 604c100..12a5117 100644
--- a/openbsc/src/libmsc/osmo_msc.c
+++ b/openbsc/src/libmsc/osmo_msc.c
@@ -173,5 +173,5 @@
 
 	conn->in_release = 1;
 	gsm0808_clear(conn);
-	subscr_con_free(conn);
+	msc_subscr_con_free(conn);
 }