Merge _PyCoreConfig_ReadEnv() into _PyCoreConfig_Read(), and
_Py_CommandLineDetails usage is now restricted to pymain_cmdline().

Changes:

* _PyCoreConfig: Add nxoption, xoptions, nwarnoption and warnoptions
* Add _PyCoreConfig.program: argv[0] or ""
* Move filename, command, module and xoptions from
  _Py_CommandLineDetails to _PyMain. xoptions _Py_OptList becomes
  (int, wchar_t**) list.
* Add pymain_cmdline() function
* Rename copy_argv() to copy_wstrlist(). Rename clear_argv() to
  clear_wstrlist(). Remove _Py_OptList structure: use (int,
  wchar_t**) list instead.
* Rename pymain_set_flag_from_env() to pymain_get_env_flag()
* Rename pymain_set_flags_from_env() to pymain_get_env_flags()
* _PyMainInterpreterConfig_Read() now creates the warnoptions from
  _PyCoreConfig.warnoptions
* Inline pymain_add_warning_dev_mode() and
  pymain_add_warning_bytes_flag() into config_init_warnoptions()
* Inline pymain_get_program_name() into _PyCoreConfig_Read()
* _Py_CommandLineDetails: Replace warning_options with nwarnoption
  and warnoptions. Replace env_warning_options with nenv_warnoption
  and env_warnoptions.
* pymain_warnings_envvar() now has a single implementation for
  Windows and Unix: use config_get_env_var_dup() to also get the
  variable as wchar_t* on Unix.