This is no longer the real main program; it now defines Py_Main(), so

it can be placed in the library.

Other, related changes:

- Moved the inspection of some environment variables to
Py_Initialize().

- Got rid of -s option.

- Moved Py_GetProgramName() and related logic to pythonrun.c; call
Py_SetProgramName() instead.

- Print the version header *after* successful initialization.