bpo-33042: Fix pre-initialization sys module configuration (GH-6157)

- new test case for pre-initialization of sys.warnoptions and sys._xoptions
- restored ability to call these APIs prior to Py_Initialize
- updated the docs for the affected APIs to make it clear they can be
  called before Py_Initialize
- also enhanced the existing embedding test cases
  to check for expected settings in the sys module

Doc/c-api/init.rst

@@ -31,6 +31,9 @@ The following functions can be safely called before Python is initialized:
   * :c:func:`Py_SetProgramName`
   * :c:func:`Py_SetPythonHome`
   * :c:func:`Py_SetStandardStreamEncoding`
+  * :c:func:`PySys_AddWarnOption`
+  * :c:func:`PySys_AddXOption`
+  * :c:func:`PySys_ResetWarnOptions`
 
 * Informative functions:
 

Doc/c-api/sys.rst

@@ -205,16 +205,24 @@ accessible to C code.  They all work with the current interpreter thread's
 
 .. c:function:: void PySys_ResetWarnOptions()
 
-   Reset :data:`sys.warnoptions` to an empty list.
+   Reset :data:`sys.warnoptions` to an empty list. This function may be
+   called prior to :c:func:`Py_Initialize`.
 
 .. c:function:: void PySys_AddWarnOption(const wchar_t *s)
 
-   Append *s* to :data:`sys.warnoptions`.
+   Append *s* to :data:`sys.warnoptions`. This function must be called prior
+   to :c:func:`Py_Initialize` in order to affect the warnings filter list.
 
 .. c:function:: void PySys_AddWarnOptionUnicode(PyObject *unicode)
 
    Append *unicode* to :data:`sys.warnoptions`.
 
+   Note: this function is not currently usable from outside the CPython
+   implementation, as it must be called prior to the implicit import of
+   :mod:`warnings` in :c:func:`Py_Initialize` to be effective, but can't be
+   called until enough of the runtime has been initialized to permit the
+   creation of Unicode objects.
+
 .. c:function:: void PySys_SetPath(const wchar_t *path)
 
    Set :data:`sys.path` to a list object of paths found in *path* which should
@@ -260,7 +268,8 @@ accessible to C code.  They all work with the current interpreter thread's
 .. c:function:: void PySys_AddXOption(const wchar_t *s)
 
    Parse *s* as a set of :option:`-X` options and add them to the current
-   options mapping as returned by :c:func:`PySys_GetXOptions`.
+   options mapping as returned by :c:func:`PySys_GetXOptions`. This function
+   may be called prior to :c:func:`Py_Initialize`.
 
    .. versionadded:: 3.2