CLEANUP: stconn: rename cs_mux() to sc_mux_strm()

The function doesn't return a pointer to the mux but to the mux stream
(h1s, h2s etc). Let's adjust its name to reflect this. It's rarely used,
the name can be enlarged a bit. And of course s/cs/sc to accommodate for
the updated name.
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 9b37d05..a993a5a 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -1458,7 +1458,7 @@
 static size_t qc_rcv_buf(struct stconn *cs, struct buffer *buf,
                          size_t count, int flags)
 {
-	struct qcs *qcs = __cs_mux(cs);
+	struct qcs *qcs = __sc_mux_strm(cs);
 	struct htx *qcs_htx = NULL;
 	struct htx *cs_htx = NULL;
 	size_t ret = 0;
@@ -1528,7 +1528,7 @@
 static size_t qc_snd_buf(struct stconn *cs, struct buffer *buf,
                          size_t count, int flags)
 {
-	struct qcs *qcs = __cs_mux(cs);
+	struct qcs *qcs = __sc_mux_strm(cs);
 	size_t ret;
 
 	TRACE_ENTER(QMUX_EV_STRM_SEND, qcs->qcc->conn, qcs);
@@ -1548,7 +1548,7 @@
 static int qc_subscribe(struct stconn *cs, int event_type,
                         struct wait_event *es)
 {
-	return qcs_subscribe(__cs_mux(cs), event_type, es);
+	return qcs_subscribe(__sc_mux_strm(cs), event_type, es);
 }
 
 /* Called from the upper layer, to unsubscribe <es> from events <event_type>.
@@ -1557,7 +1557,7 @@
  */
 static int qc_unsubscribe(struct stconn *cs, int event_type, struct wait_event *es)
 {
-	struct qcs *qcs = __cs_mux(cs);
+	struct qcs *qcs = __sc_mux_strm(cs);
 
 	BUG_ON(event_type & ~(SUB_RETRY_SEND|SUB_RETRY_RECV));
 	BUG_ON(qcs->subs && qcs->subs != es);