bpo-34170: Cleanup pymain_read_conf() (GH-8476)

* Config: Rename ignore_environment field to use_environment.
* _PyCoreConfig_Read(): if isolated is set, use_environment and
  site_import are now always set to 0.
* Inline pymain_free_raw() into pymain_free()
* Move config_init_warnoptions() call into pymain_read_conf_impl()
* _PyCoreConfig_Read(): don't replace values if they are already set:
  faulthandler, pycache_prefix, home.