Fix a crash with musl libc (on Alpine Linux) when the script filename
specified on the command line doesn't exist. pymain_open_filename()
now gets the current core configuration from the interpreter state.

Modify the code to make it closer to the master branch:

* Rename _Py_CommandLineDetails to _PyCmdline
* Remove _PyMain.config: replaced with a local variable
  'local_config' in pymain_init()
* Reorganize pymain_main(): move code using the "local config"
  into pymain_init()
* As soon as possible, switch from the local config to the core
  configuration attached to the interpreter.