EXPERIMENTAL: Dedicated server idling system

- If no clients at server start or after 10 seconds of GS_LEVEL, and no Netxcmd waiting to be digested, halt all SV_MakeTic.
- It's absolutely netsafe to only have enabled on the host's end, the only risk is that a dedicated server might not re-awaken when presented with certain stimuli

src/d_clisrv.c

@@ -5767,6 +5767,9 @@ void NetKeepAlive(void)
 	FileSendTicker();
 }
 
+// If a tree falls in the forest but nobody is around to hear it, does it make a tic?
+#define DEDICATEDIDLETIME (10*TICRATE)
+
 void NetUpdate(void)
 {
 	static tic_t resptime = 0;
@@ -5779,6 +5782,7 @@ void NetUpdate(void)
 
 	if (realtics <= 0) // nothing new to update
 		return;
+
 	if (realtics > 5)
 	{
 		if (server)
@@ -5787,6 +5791,55 @@ void NetUpdate(void)
 			realtics = 5;
 	}
 
+#ifdef DEDICATEDIDLETIME
+	if (server && dedicated && gamestate == GS_LEVEL)
+	{
+		static tic_t dedicatedidle = 0;
+
+		for (i = 1; i < MAXNETNODES; ++i)
+			if (nodeingame[i])
+			{
+				if (dedicatedidle == DEDICATEDIDLETIME)
+				{
+					CONS_Printf("DEDICATED: Awakening from idle (Node %d detected...)\n", i);
+					dedicatedidle = 0;
+				}
+				break;
+			}
+
+		if (i == MAXNETNODES)
+		{
+			if (leveltime == 2)
+			{
+				// On next tick...
+				dedicatedidle = DEDICATEDIDLETIME-1;
+			}
+			else if (dedicatedidle == DEDICATEDIDLETIME)
+			{
+				if (D_GetExistingTextcmd(gametic, 0) || D_GetExistingTextcmd(gametic+1, 0))
+				{
+					CONS_Printf("DEDICATED: Awakening from idle (Netxcmd detected...)\n");
+					dedicatedidle = 0;
+				}
+				else
+				{
+					realtics = 0;
+				}
+			}
+			else if ((dedicatedidle += realtics) >= DEDICATEDIDLETIME)
+			{
+				const char *idlereason = "at round start";
+				if (leveltime > 3)
+					idlereason = va("for %d seconds", dedicatedidle/TICRATE);
+
+				CONS_Printf("DEDICATED: No nodes %s, idling...\n", idlereason);
+				realtics = 0;
+				dedicatedidle = DEDICATEDIDLETIME;
+			}
+		}
+	}
+#endif
+
 	gametime = nowtime;
 
 	UpdatePingTable();
@@ -5824,7 +5877,7 @@ void NetUpdate(void)
 	}
 	else
 	{
-		if (!demo.playback)
+		if (!demo.playback && realtics > 0)
 		{
 			INT32 counts;