lib: eliminate size_t casts

Add new functions in `curlx/warnless.h` for controlled type
conversions:

* curlx_uitouz, convert unsigned into to size_t (should always work)
* curlx_uztoso, convert size_t to curl_off_t, capping at CURL_OFF_T_MAX
* curlx_sztouz, convert ssize_t to size_t, return TRUE when ok
* curlx_sotouz_range, convert curl_off_t to size_t interval, capping
  values to interval bounds

Remove some unnecesary casts, convert some internal recv functions
to the "return result, have size_t* arg" pattern.

Closes #19495

lib/cf-h2-proxy.c

@@ -847,7 +847,7 @@ static int tunnel_recv_callback(nghttp2_session *session, uint8_t flags,
 #endif
   }
   /* tunnel.recbuf has soft limit, any success MUST add all data */
-  DEBUGASSERT((size_t)nwritten == len);
+  DEBUGASSERT(nwritten == len);
   return 0;
 }