MEDIUM: conn_stream: add cs_recv() as a default rcv_buf() function

This function is generic and is able to automatically transfer data
from a conn_stream's rx buffer to the destination buffer. It does this
automatically if the mux doesn't define another rcv_buf() function.
diff --git a/src/checks.c b/src/checks.c
index af6ddd4..b965ee6 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -850,7 +850,7 @@
 
 	done = 0;
 
-	conn->mux->rcv_buf(cs, &check->bi, b_size(&check->bi), 0);
+	cs_recv(cs, &check->bi, b_size(&check->bi), 0);
 	if (conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH) || cs->flags & CS_FL_ERROR) {
 		done = 1;
 		if ((conn->flags & CO_FL_ERROR || cs->flags & CS_FL_ERROR) && !b_data(&check->bi)) {
@@ -2918,7 +2918,7 @@
 				goto out_end_tcpcheck;
 
 			__cs_want_recv(cs);
-			if (conn->mux->rcv_buf(cs, &check->bi, b_size(&check->bi), 0) <= 0) {
+			if (cs_recv(cs, &check->bi, b_size(&check->bi), 0) <= 0) {
 				if (conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH) || cs->flags & CS_FL_ERROR) {
 					done = 1;
 					if ((conn->flags & CO_FL_ERROR || cs->flags & CS_FL_ERROR) && !b_data(&check->bi)) {