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/libtest/lib558.c

@@ -39,7 +39,7 @@ static CURLcode test_lib558(const char *URL)
   }
 
   ptr = curlx_malloc(558);
-  Curl_safefree(ptr);
+  curlx_safefree(ptr);
 
   asize = (int)sizeof(a);
   ptr = curl_easy_escape(NULL, (char *)a, asize);