- Fix 'my_win_init' code that handles initialization of
   "query_performance_counter" and conversion from registry values to
   environment strings


mysys/my_init.c:
  - Change check of return value from 'QueryPerformanceFrequency', it
    returns 0 on error
  - Don't return from 'my_win_init' if the registry 
    key "HKEY_LOCAL_MACHINE\SOFTWARE\MySQL" doesn't exist. This caused
    the initialization of "query_performance_counter" to be bypassed unless there
    was at lest such a key.
  - Cleanup the "registry -> environment variables" from Italian comments,
    remove redundant variables and fix style.
  - Remove function 'setEnvString' and replace it with oneliner 'strxmov'