windows: bug fixes of uv_signal_init/start and new support of signal handling in uv_kill()

This PR has the following three commits that have two bug fixes and one new feature. 

+ bug fix: `uv__signal_control_handler_refs` needs to be incremented when multiple signal handlers are started.

+ bug fix: `uv__handle_start()` was not needed in `uv_signal_init()`

+ new feature: support signal handling in uv_kill(). This is nice to have consistent behaviors between Windows and Unix.