[NTVDM]

- Enable experimental sound support (only PC speaker for the moment, aka. uses beep.sys).
- Introduce a #define WORKING_TIMER which aim is to disable the currently problematic approximate performance counter value calculation done in order not to call QueryPerformanceCounter each time.
  The problem is that we then compute a number of clock ticks for the PIT, which becomes negative, and therefore everything starts to hang.
  Disabling this code and calling each time QueryPerformanceCounter, fixes everything; we gain in precision but we loose in performance...
  A definitive fix must be found, [TheFlash] !!

This fixes sound (and hangs) in Advanced NetWars, Dangerous Dave, ElitePlus and Rescue Rover (the games that I've tested so far).

svn path=/branches/ntvdm/; revision=61875