feat(headless): complete portable single-session host

src/AcDream.Headless/Program.cs

@@ -1,3 +1,36 @@
+using System.Runtime.InteropServices;
 using AcDream.Headless;
 
-return HeadlessEntryPoint.Run(args, Console.Out, Console.Error);
+using var cancellation = new CancellationTokenSource();
+ConsoleCancelEventHandler cancel = (_, eventArgs) =>
+{
+    eventArgs.Cancel = true;
+    cancellation.Cancel();
+};
+Console.CancelKeyPress += cancel;
+PosixSignalRegistration? terminate = null;
+if (!OperatingSystem.IsWindows())
+{
+    terminate = PosixSignalRegistration.Create(
+        PosixSignal.SIGTERM,
+        context =>
+        {
+            context.Cancel = true;
+            cancellation.Cancel();
+        });
+}
+
+try
+{
+    return HeadlessEntryPoint.Run(
+        args,
+        Console.In,
+        Console.Out,
+        Console.Error,
+        cancellation.Token);
+}
+finally
+{
+    terminate?.Dispose();
+    Console.CancelKeyPress -= cancel;
+}