dynbuf: make *addf() not require extra mallocs

... by introducing a printf() function that appends directly into a
dynbuf: Curl_dyn_vprintf(). This avoids the mandatory extra malloc so if
the buffer is already big enough it can just printf directly into it.

Since this less-malloc version requires tthe use of a library internal
printf function, we only provide this version when building libcurl and
not for the dynbuf code that is used when building the curl tool.

Closes #5998

lib/dynbuf.c

@@ -181,8 +181,17 @@ CURLcode Curl_dyn_add(struct dynbuf *s, const char *str)
  */
 CURLcode Curl_dyn_addf(struct dynbuf *s, const char *fmt, ...)
 {
-  char *str;
   va_list ap;
+#ifdef BUILDING_LIBCURL
+  int rc;
+  va_start(ap, fmt);
+  rc = Curl_dyn_vprintf(s, fmt, ap);
+  va_end(ap);
+
+  if(!rc)
+    return CURLE_OK;
+#else
+  char *str;
   va_start(ap, fmt);
   str = vaprintf(fmt, ap); /* this allocs a new string to append */
   va_end(ap);
@@ -194,9 +203,12 @@ CURLcode Curl_dyn_addf(struct dynbuf *s, const char *fmt, ...)
   }
   /* If we failed, we cleanup the whole buffer and return error */
   Curl_dyn_free(s);
+#endif
   return CURLE_OUT_OF_MEMORY;
 }
 
+
+
 /*
  * Returns a pointer to the buffer.
  */