bpo-43963: Add _signal module state (GH-25676)

* Add signal_state_t structure and signal_global_state variable.
* Add a module state to the _signal module.
* Move and rename variables:

  * DefaultHandler becomes state->default_handler
  * IgnoreHandler becomes state->ignore_handler
  * sigint_event becomes state->sigint_event
  * ItimerError becomes modstate->itimer_error

* Rename SetHandler() to set_handler() to be consistent with
  get_handler().

Author: vstinner