Rewrite Win32 (as opposed to Win32s) DLL to use auxiliary thread and

Rewrite Win32 (as opposed to Win32s) DLL to use auxiliary thread and
Sleep() API call to generate async timer interrupts.  Use a critical
section to synchronize the two threads' access to the interrupt
registers.  These changes should eliminate the "spurious interrupt"
error messages that have been showing up from time to time.