MINOR: tree-wide: Replace several chn_cons() by the corresponding SC

At many places, call to chn_cons() can be easily replaced by the
corresponding SC. It is a bit easier to understand which side is
manipulated.
diff --git a/src/stconn.c b/src/stconn.c
index d320173..eca1e45 100644
--- a/src/stconn.c
+++ b/src/stconn.c
@@ -1369,7 +1369,7 @@
 		cur_read += ret;
 
 		/* if we're allowed to directly forward data, we must update ->o */
-		if (ic->to_forward && !(chn_cons(ic)->flags & (SC_FL_SHUT_DONE|SC_FL_SHUT_WANTED))) {
+		if (ic->to_forward && !(sc_opposite(sc)->flags & (SC_FL_SHUT_DONE|SC_FL_SHUT_WANTED))) {
 			unsigned long fwd = ret;
 			if (ic->to_forward != CHN_INFINITE_FORWARD) {
 				if (fwd > ic->to_forward)