bpo-41713: Remove PyOS_InitInterrupts() function (GH-23342)

Remove the undocumented PyOS_InitInterrupts() C function.

* Rename PyOS_InitInterrupts() to _PySignal_Init(). It now installs
  other signal handlers, not only SIGINT.
* Rename PyOS_FiniInterrupts() to _PySignal_Fini()