Rename PyUnicode_AsString -> _PyUnicode_AsString and
PyUnicode_AsStringAndSize -> _PyUnicode_AsStringAndSize to mark
them for interpreter internal use only.

We'll have to rework these APIs or create new ones for the
purpose of accessing the UTF-8 representation of Unicode objects
for 3.1.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 4cdaf25..3b79d05 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -5566,7 +5566,7 @@
                             "configuration names must be strings or integers");
             return 0;
         }
-        confname = PyUnicode_AsString(arg);
+        confname = _PyUnicode_AsString(arg);
         if (confname == NULL)
             return 0;
         while (lo < hi) {
@@ -5897,7 +5897,7 @@
 	    if ((unsigned int)len >= sizeof(buffer)) {
                 result = PyUnicode_FromStringAndSize(NULL, len-1);
                 if (result != NULL)
-                    confstr(name, PyUnicode_AsString(result), len);
+                    confstr(name, _PyUnicode_AsString(result), len);
             }
             else
                 result = PyUnicode_FromStringAndSize(buffer, len-1);