lib: use Curl_str_number() for parsing decimal numbers

Instead of strtoul() and strtol() calls.

Easier API with better integer overflow detection and built-in max check
that now comes automatic everywhere this is used.

Closes #16319

lib/pingpong.h

@@ -69,7 +69,7 @@ struct pingpong {
 
   CURLcode (*statemachine)(struct Curl_easy *data, struct connectdata *conn);
   bool (*endofresp)(struct Curl_easy *data, struct connectdata *conn,
-                    char *ptr, size_t len, int *code);
+                    const char *ptr, size_t len, int *code);
 };
 
 #define PINGPONG_SETUP(pp,s,e)                   \