dissolve libcommon: move talloc ctx into msc_main.c, drop talloc_ctx.c

Drop tall_bsc_ctx; in mncc_sock_init(), talloc the mncc_sock_state from
gsm_network.

In tests or utils, move from using an extern tall_bsc_ctx to a local root
context pointer.

Change-Id: I92c252be1d1e7634f1653de47d37c99d77d9501c
diff --git a/src/libmsc/mncc_sock.c b/src/libmsc/mncc_sock.c
index 14613ca..74ea9d2 100644
--- a/src/libmsc/mncc_sock.c
+++ b/src/libmsc/mncc_sock.c
@@ -284,7 +284,7 @@
 	struct osmo_fd *bfd;
 	int rc;
 
-	state = talloc_zero(tall_bsc_ctx, struct mncc_sock_state);
+	state = talloc_zero(net, struct mncc_sock_state);
 	if (!state)
 		return -ENOMEM;