- restore startup game state on restart

At least values of custom server CVARs were not restored because of this

https://forum.zdoom.org/viewtopic.php?t=60711
(cherry picked from commit 36779cf88c)

src/d_main.cpp

@@ -2823,6 +2823,8 @@ void D_DoomMain (void)
 		restart++;
 		PClass::bShutdown = false;
 		PClass::bVMOperational = false;
+
+		gamestate = GS_STARTUP;
 	}
 	while (1);
 }