let the CPU sleep for 1 ms if there is no tic to proceed

This brings CPU usage down from 100% to ~18% on my system - just like
Chocolate Doom.

Source/d_net.c

@@ -758,6 +758,7 @@ void TryRunTics (void)
       M_Ticker ();
       return;
     } 
+    I_Sleep(1);
   }
   
   // run the count * ticdup dics

Source/i_system.c

@@ -59,6 +59,11 @@ void I_WaitVBL(int count)
    SDL_Delay((count*500)/TICRATE);
 }
 
+void I_Sleep(int ms)
+{
+    SDL_Delay(ms);
+}
+
 // Most of the following has been rewritten by Lee Killough
 //
 // I_GetTime

Source/i_video.h

@@ -47,6 +47,7 @@ void I_FinishUpdate (void);
 
 // Wait for vertical retrace or pause a bit.
 void I_WaitVBL(int count);
+void I_Sleep(int ms);
 
 void I_ReadScreen (byte* scr);