trap stack: update special handling for SIGINT on interactive bash
trap stack: update special handling for SIGINT on interactive bash

bin/modernish:
- The bug that kills an interactive bash shell upon resending
  SIGINT from an INT trap handler has been fixed in bash
  5.0.0-alpha. Since it is not possible to implement a shell bug
  test for this, we have to do ugly version checking to disable the
  workaround for bash 5.0 and up. Also move the workaround to a
  smarter place and update/abbreviate the comments.

modernish/modernish