Use a debug system property to control debugging.

When set, it disables stdout/stderr redirection and uses java.exe on Windows so
that a console window is shown with output from the launched app.

This removes the use of a debug.txt file.

core/src/main/java/com/threerings/getdown/data/Application.java

@@ -784,10 +784,6 @@ public class Application
         // extract some info used to configure our child process on macOS
         _dockName = config.getString("ui.name");
         _dockIconPath = config.getString("ui.mac_dock_icon", "../desktop.icns");
-
-        // look for a debug.txt file which causes us to run in java.exe on Windows so that we can
-        // obtain a thread dump of the running JVM
-        _windebug = getLocalPath("debug.txt").exists();
     }
 
     /**
@@ -967,7 +963,7 @@ public class Application
         ArrayList<String> args = new ArrayList<>();
 
         // reconstruct the path to the JVM
-        args.add(LaunchUtil.getJVMBinaryPath(_javaLocalDir, _windebug || optimum));
+        args.add(LaunchUtil.getJVMBinaryPath(_javaLocalDir, SysProps.debug() || optimum));
 
         // check whether we're using -jar mode or -classpath mode
         boolean dashJarMode = MANIFEST_CLASS.equals(_class);
@@ -1737,7 +1733,6 @@ public class Application
     protected String _dockName;
     protected String _dockIconPath;
     protected boolean _strictComments;
-    protected boolean _windebug;
     protected boolean _allowOffline;
     protected int _maxConcDownloads;
 

core/src/main/java/com/threerings/getdown/data/SysProps.java

@@ -40,6 +40,14 @@ public final class SysProps
         return System.getProperty("no_log_redir") != null;
     }
 
+    /** Used to debug Getdown's launching of an app. When set, it disables redirection of stdout
+      * and stderr into a log file, and on Windows it uses {@code java.exe} to launch the app so
+      * that its console output can be observed.
+      * Usage: {@code -Ddebug}. */
+    public static boolean debug () {
+        return System.getProperty("debug") != null;
+    }
+
     /** Overrides the domain on {@code appbase}. Usage: {@code -Dappbase_domain=foo}. */
     public static String appbaseDomain () {
         return System.getProperty("appbase_domain");