[vty] Move db usage out of the libbsc (fork dumping of subscr)

vty_interface.c is part of libbsc.a but it started to use code
which is found in db.c recently. Fork the subscriber dumping and
provide more information on the layer3+ (MSC) commands. This
is restoring the separation again.
diff --git a/openbsc/src/vty_interface.c b/openbsc/src/vty_interface.c
index c9a5c17..8301f78 100644
--- a/openbsc/src/vty_interface.c
+++ b/openbsc/src/vty_interface.c
@@ -512,7 +512,7 @@
 	return CMD_SUCCESS;
 }
 
-void subscr_dump_vty(struct vty *vty, struct gsm_subscriber *subscr)
+static void subscr_dump_vty(struct vty *vty, struct gsm_subscriber *subscr)
 {
 	int rc;
 	struct gsm_auth_info ainfo;