Fix ngx 0.5.0 API compatibility

- Update HttpModule trait implementation to match ngx 0.5.0 API
- Implement HttpModuleLocationConf as separate unsafe trait
- Fix configuration access using Module::location_conf()
- Replace ngx_null_command macro with explicit null command
- Update imports to use correct constant names
- Suppress C FFI naming convention warnings

conf/howto.conf

@@ -1,5 +1,5 @@
 # Add the path to your library here.
-load_module target/debug/libnginx_test.so;
+load_module target/debug/libnginx_test.dylib;
 
 worker_processes 1;