tidy-up: rename Curl_safefree()/tool_safefree() to curlx_safefree()

To sync names for the same macro logic between lib and src, and to move
it to the curlx namespace, to match `curlx_free()` that it's calling.

Closes #21151

tests/unit/unit1330.c

@@ -28,7 +28,7 @@ static CURLcode test_unit1330(const char *arg)
   UNITTEST_BEGIN_SIMPLE
 
   char *ptr = curlx_malloc(1330);
-  Curl_safefree(ptr);
+  curlx_safefree(ptr);
 
   UNITTEST_END_SIMPLE
 }