Verify SHA of downloaded net file

check SHA of the available and downloaded file.

Document the format requirement on the default net.

Also allow curl to make possibly insecure connections, as needed for old curl.

fixes https://github.com/official-stockfish/Stockfish/issues/2998

closes https://github.com/official-stockfish/Stockfish/pull/3000

No functional change.

src/ucioption.cpp

@@ -79,6 +79,8 @@ void init(OptionsMap& o) {
   o["Syzygy50MoveRule"]      << Option(true);
   o["SyzygyProbeLimit"]      << Option(7, 0, 7);
   o["Use NNUE"]              << Option(false, on_use_NNUE);
+  // The default must follow the format nn-[SHA256 first 12 digits].nnue
+  // for the build process (profile-build and fishtest) to work.
   o["EvalFile"]              << Option("nn-82215d0fd0df.nnue", on_eval_file);
 }