bpo-41686: Always create the SIGINT event on Windows (GH-23344)

bpo-41686, bpo-41713: On Windows, the SIGINT event,
_PyOS_SigintEvent(), is now created even if Python is configured to
not install signal handlers (PyConfig.install_signal_handlers=0 or
Py_InitializeEx(0)).

Changes:

* Move global variables initialization from signal_exec() to
  _PySignal_Init() to clarify that they are global variables cleared
  by _PySignal_Fini().
* _PySignal_Fini() now closes sigint_event.
* IntHandler is no longer a global variable.