- reset the network timer after lengthy operations.

This includes loading a level and busy-waiting for a sound to play.
Also block these loops and the sounds they wait for in network games to avoid problems from longer delays here.
The problem seems to be directly inherited from ZDoom which shows the same issue with screen wipes.

Fixes #297

source/core/d_net.cpp

@@ -1972,9 +1972,12 @@ void Net_SkipCommand (int type, uint8_t **stream)
 #endif
 }
 
+// Reset the network ticker after finishing a lengthy operation.
+// Q: How does this affect network sync? Only allowed in SP games?
 void Net_ClearFifo(void)
 {
-	// Q: Do we need this?
+	I_SetFrameTime();
+	gametime = I_GetTime();
 }