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/imap.c

@@ -80,9 +80,7 @@
 #include "rawstr.h"
 #include "curl_sasl.h"
 #include "warnless.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: */