mprintf.h: remove #ifdef CURLDEBUG

... and as a consequence, introduce curl_printf.h with that re-define
magic instead and make all libcurl code use that instead.

lib/multi.c

@@ -43,10 +43,7 @@
 #include "multihandle.h"
 #include "pipeline.h"
 #include "sigpipe.h"
-
-#define _MPRINTF_REPLACE /* use our functions only */
-#include <curl/mprintf.h>
-
+#include "curl_printf.h"
 #include "curl_memory.h"
 /* The last #include file should be: */
 #include "memdebug.h"