* synchronous signal overlaps its function with another inter-thread communication
  feature (like msgq,barrier), so it's removed.
  however, asynchronous signal could be very useful handle asynchronous kernel (or hal) event
* Asynchronous Signal is implemented as draft
  - will allow generate kernel event asynchronously
  - will handled by signal handler in user mode
  - it's a little bit similar to unix, except..
     - support signal arguement which can be accessed in signal handler
     - per-thread signal handler (which was per-process in unix)
     - 4 types of signal used (is very minimal compared to unix or linux set)