feat: optional restart exit code so a supervisor owns the relaunch (#2705)

Fixes #2700 (with #2704)

A restart (`restart` console command, `AutoRestart`, `CrashGuard`) launches a copy of the server and exits. That copy is detached from the console it was started from: the shell takes the terminal back and console commands stop working. Under systemd or in a container it's killed as soon as the original process exits.

- New setting `server.restartExitCode` (off by default). When it's set, a restart saves and shuts down as usual and then exits with that code, so a shell loop, systemd unit or container restart policy can start the server again on the same console. Values outside 1–255 log a warning and fall back to the current behaviour.
- Fixes `CrashGuard` crash recovery never restarting the server. Since #1809, `Kill()` only records the request, and the unhandled-exception path called `DoKill()` without it.
- Documents systemd, Docker and shell-loop setups in `dev-docs/server-requirements.md`.

Projects/UOContent/Misc/CrashGuard.cs

@@ -13,7 +13,7 @@ namespace Server.Misc
 
         private static bool Enabled;
         private static bool SaveBackup;
-        private static bool RestartServer; // Disable this if using a daemon/service
+        private static bool RestartServer; // Under a daemon/service, set server.restartExitCode rather than disabling this
         private static bool GenerateReport;
 
         public static void Configure()