Hook SIGUSR2 to requestStop (#842)

For macOS this brings up a dialog, asking the user if they are sure
they want to shut down, which makes this less useful for automated
graceful shutdowns, but it may behave better on Linux, and there
might be ways to instruct macOS to not ask the user, so it's still
a nice feature, and aligns with the SIGUSR1 for suspend, and SIGINT
for non-graceful shutdown.

Sources/tart/Commands/Run.swift

@@ -359,6 +359,18 @@ struct Run: AsyncParsableCommand {
     }
     sigusr1Src.activate()
 
+    // Gracefull shutdown support. For macOS this brings up a dialog,
+    // asking the user if they are sure they want to shut down.
+    signal(SIGUSR2, SIG_IGN)
+    let sigusr2Src = DispatchSource.makeSignalSource(signal: SIGUSR2)
+    sigusr2Src.setEventHandler {
+      Task {
+        print("Requesting guest OS to stop...")
+        try vm!.virtualMachine.requestStop()
+      }
+    }
+    sigusr2Src.activate()
+
     let useVNCWithoutGraphics = (vnc || vncExperimental) && !graphics
     if noGraphics || useVNCWithoutGraphics {
       // enter the main even loop, without bringing up any UI,