Add PYTHON_API_STRING, which is PYTHON_API_VERSION as a string literal.

Add PYTHON_API_STRING, which is PYTHON_API_VERSION as a string literal.
Under Windows, add MS_DLL_ID and MS_DLL_VERSION_ID for Mark H.

Independent change: if Py_TRACE_REFS is defined, rename Py_InitModule4
so so linking with incompatible modules will create a link time error.

[Backing out of previous changes (also for modsupport.c) to test
the latter at runtime.]