graceful shutdown: ./shutdown flushes players, then stops the servers

The stock scripts stop the game with SIGKILL (mbkill.sh runs kill -9) and
'./reboot' only calls mbrestart.sh, so anything still parked in a character's
deferred database job is thrown away: experience is written five minutes after
it is earned and stats/skills thirty seconds after they change
(engine.jobs.DatabaseUpdateJob). PlayerCharacter.updateDatabase() is an empty
stub, so there was no write-out path at all.

Added engine.gameManager.GracefulShutdown:
  flushAllPlayers()      - writes skills/powers, stat modifiers and experience
                           for every online character
  announce(String)       - server wide flash message
  registerShutdownHook() - flush on an orderly SIGTERM (SIGKILL cannot be
                           intercepted by design)
  gracefulStop(int)      - countdown, flush, then stop the login and world
                           servers

Added the './shutdown [seconds]' dev command (engine.devcmd.cmds.ShutdownCmd,
default 60s, ADMIN only like every other dev command), registered together with
the shutdown hook in WorldServer.main and LoginServer.main.

Verified on a MagicBox container: ant builds clean, the boot log shows
"shutdown hook registered for WorldServer"/"LoginServer", and kill -TERM on the
world server logs "WorldServer is terminating, flushing player data" ->
"flush finished, saved=0 failed=0" -> clean exit.

src/engine/server/login/LoginServer.java

@@ -96,6 +96,11 @@ public class LoginServer {
             // Start the Login Server
 
             loginServer.init();
+
+            // flush any cached character state on an orderly termination
+            // (the login server holds no players, so this normally logs 0).
+
+            GracefulShutdown.registerShutdownHook("LoginServer");
             loginServer.exec();
 
             exit(0);