MINOR: connections: Introduce an unsubscribe method.

As we don't know how subscriptions are handled, we can't just assume we can
use LIST_DEL() to unsubscribe, so introduce a new method to mux and connections
to do so.
diff --git a/src/raw_sock.c b/src/raw_sock.c
index c108a42..df861f4 100644
--- a/src/raw_sock.c
+++ b/src/raw_sock.c
@@ -425,6 +425,7 @@
 	.snd_buf  = raw_sock_from_buf,
 	.rcv_buf  = raw_sock_to_buf,
 	.subscribe = conn_subscribe,
+	.unsubscribe = conn_unsubscribe,
 #if defined(CONFIG_HAP_LINUX_SPLICE)
 	.rcv_pipe = raw_sock_to_pipe,
 	.snd_pipe = raw_sock_from_pipe,