Fix Windows shutdown hang and register console ctrl handler

- Remove GenerateBitcoins(false) from stop RPC handler because it calls
  minerThreads->join_all() which blocks the RPC thread, preventing the
  response from being sent back to the client. Miners are already stopped
  in Shutdown() so this was redundant.
- Add ShutdownRequested() checks in miner peer-wait and nonce loops
- Register SetConsoleCtrlHandler on Windows for clean Ctrl+C/close
- Add check_and_clean_target for --linux-release builds

build.sh

@@ -43,6 +43,7 @@ fi
 
 # Check for --linux-release flag (builds inside Ubuntu 20.04 container for max compatibility)
 if [[ "${1:-}" == "--linux-release" ]]; then
+    check_and_clean_target "linux-docker"
     shift
     echo "Building Linux release inside Ubuntu 20.04 Docker container..."
     sudo docker build -t hush-builder -f Dockerfile.build .