bpo-34170: Enhance _PyCoreConfig_Read() (GH-8468)

* Inline cmdline_get_env_flags() into config_read_env_vars():
  _PyCoreConfig_Read() now reads much more environment variables like
  PYTHONVERBOSE.
* Allow to override faulthandler and allocator even if dev_mode=1.
  PYTHONMALLOC is now the priority over PYTHONDEVMODE.
* Fix _PyCoreConfig_Copy(): copy also install_signal_handlers,
  coerce_c_locale and coerce_c_locale_warn
* _PyCoreConfig.install_signal_handlers default is now 1: install
  signals by default
* Fix also a compiler warning: don't define _PyPathConfig type twice.