multi.h: move INITIAL_MAX_CONCURRENT_STREAMS from public header

... to the private multihhandle.h. It is not for public use and it
wasn't prefixed correctly anyway!

Closes #4790

include/curl/multi.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -461,9 +461,6 @@ typedef int (*curl_push_callback)(CURL *parent,
                                   struct curl_pushheaders *headers,
                                   void *userp);
 
-/* value for MAXIMUM CONCURRENT STREAMS upper limit */
-#define INITIAL_MAX_CONCURRENT_STREAMS ((1U << 31) - 1)
-
 #ifdef __cplusplus
 } /* end of extern "C" */
 #endif

lib/multihandle.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -71,6 +71,10 @@ typedef enum {
 #define ENABLE_WAKEUP
 #endif
 
+
+/* value for MAXIMUM CONCURRENT STREAMS upper limit */
+#define INITIAL_MAX_CONCURRENT_STREAMS ((1U << 31) - 1)
+
 /* This is the struct known as CURLM on the outside */
 struct Curl_multi {
   /* First a simple identifier to easier detect if a user mix up