`Core.DoKill(true)` built the relaunch command line by putting the assembly path into a raw argument string without quotes, so `dotnet` split it at the first space. That breaks `restart`, `AutoRestart` and `CrashGuard` recovery for any server installed under a path with a space.
- Pass the assembly path through `ProcessStartInfo.ArgumentList`, which quotes it.
- Relaunch through the host that started the process (`Environment.ProcessPath`): the `ModernUO` apphost runs directly, and `dotnet` gets the assembly path. Before this, a server started from the apphost came back under `dotnet`.
- Use one code path on all platforms. `UseShellExecute = true` on Unix made no difference for a program found on PATH.
This doesn't fix the second half of #2700, where the relaunched server loses the console (and gets killed under systemd or Docker). That comes from the server spawning a copy of itself and exiting, and a follow-up PR will add an opt-in restart exit code for a supervisor to act on.
**Testing:** relaunched from a directory with spaces, started both through `dotnet x.dll` and through the apphost, on Windows. The old code fails there with the error from the issue.
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