lib: move signal event handling into bootstrap/node.js

Moves the `process.on()` and `promise.emit()` calls happened during
bootstrap for signal events into `bootstrap/node.js` so it's easier
to tell the side effects.

Drive-by changes:

- Moves the signal event re-arming to a point later during
  the bootstrap - as early as it were it's unlikely that there
  could be any existing signal events to re-arm for node-report.
- Use a Map instead of an Object for signal wraps since it is used
  as a deletable dictionary anyway.

PR-URL: https://github.com/nodejs/node/pull/25859


Reviewed-By: Anna Henningsen <anna@addaleax.net>