MINOR: tree-wide: Simplifiy some tests on SHUT flags by accessing SCs directly

At many places, we simplify the tests on SHUT flags to remove calls to
chn_prod() or chn_cons() function because the corresponding SC is available.
diff --git a/src/cli.c b/src/cli.c
index 21d6620..1b4276f 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1214,7 +1214,7 @@
 	char **var = ctx->var;
 
 	/* FIXME: Don't watch the other side !*/
-	if (unlikely(chn_cons(sc_ic(sc))->flags & SC_FL_SHUTW))
+	if (unlikely(sc_opposite(sc)->flags & SC_FL_SHUTW))
 		return 1;
 
 	chunk_reset(&trash);
@@ -1253,7 +1253,7 @@
 	int ret = 1;
 
 	/* FIXME: Don't watch the other side !*/
-	if (unlikely(chn_cons(sc_ic(sc))->flags & SC_FL_SHUTW))
+	if (unlikely(sc_opposite(sc)->flags & SC_FL_SHUTW))
 		goto end;
 
 	chunk_reset(&trash);