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_h2.c b/src/mux_h2.c
index dde18dc..4a3a701 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4694,7 +4694,7 @@
 /* shutr() called by the stream connector (mux_ops.shutr) */
 static void h2_shutr(struct stconn *cs, enum co_shr_mode mode)
 {
-	struct h2s *h2s = __cs_mux(cs);
+	struct h2s *h2s = __sc_mux_strm(cs);
 
 	TRACE_ENTER(H2_EV_STRM_SHUT, h2s->h2c->conn, h2s);
 	if (mode)
@@ -4705,7 +4705,7 @@
 /* shutw() called by the stream connector (mux_ops.shutw) */
 static void h2_shutw(struct stconn *cs, enum co_shw_mode mode)
 {
-	struct h2s *h2s = __cs_mux(cs);
+	struct h2s *h2s = __sc_mux_strm(cs);
 
 	TRACE_ENTER(H2_EV_STRM_SHUT, h2s->h2c->conn, h2s);
 	h2_do_shutw(h2s);
@@ -6379,7 +6379,7 @@
  */
 static int h2_subscribe(struct stconn *cs, int event_type, struct wait_event *es)
 {
-	struct h2s *h2s = __cs_mux(cs);
+	struct h2s *h2s = __sc_mux_strm(cs);
 	struct h2c *h2c = h2s->h2c;
 
 	TRACE_ENTER(H2_EV_STRM_SEND|H2_EV_STRM_RECV, h2c->conn, h2s);
@@ -6413,7 +6413,7 @@
  */
 static int h2_unsubscribe(struct stconn *cs, int event_type, struct wait_event *es)
 {
-	struct h2s *h2s = __cs_mux(cs);
+	struct h2s *h2s = __sc_mux_strm(cs);
 
 	TRACE_ENTER(H2_EV_STRM_SEND|H2_EV_STRM_RECV, h2s->h2c->conn, h2s);
 
@@ -6453,7 +6453,7 @@
  */
 static size_t h2_rcv_buf(struct stconn *cs, struct buffer *buf, size_t count, int flags)
 {
-	struct h2s *h2s = __cs_mux(cs);
+	struct h2s *h2s = __sc_mux_strm(cs);
 	struct h2c *h2c = h2s->h2c;
 	struct htx *h2s_htx = NULL;
 	struct htx *buf_htx = NULL;
@@ -6536,7 +6536,7 @@
  */
 static size_t h2_snd_buf(struct stconn *cs, struct buffer *buf, size_t count, int flags)
 {
-	struct h2s *h2s = __cs_mux(cs);
+	struct h2s *h2s = __sc_mux_strm(cs);
 	size_t total = 0;
 	size_t ret;
 	struct htx *htx;