Add an optional size argument to _Py_char2wchar()

_Py_char2wchar() callers usually need the result size in characters. Since it's
trivial to compute it in _Py_char2wchar() (O(1) whereas wcslen() is O(n)), add
an option to get it.
diff --git a/Modules/main.c b/Modules/main.c
index bbf695f..0c38fac 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -486,10 +486,12 @@
             /* Use utf-8 on Mac OS X */
             unicode = PyUnicode_FromString(p);
 #else
-            wchar_t *wchar = _Py_char2wchar(p);
+            wchar_t *wchar;
+            size_t len;
+            wchar = _Py_char2wchar(p, &len);
             if (wchar == NULL)
                 continue;
-            unicode = PyUnicode_FromWideChar(wchar, wcslen(wchar));
+            unicode = PyUnicode_FromWideChar(wchar, len);
             PyMem_Free(wchar);
 #endif
             if (unicode == NULL)
diff --git a/Modules/python.c b/Modules/python.c
index 9a71cd0..47685a4 100644
--- a/Modules/python.c
+++ b/Modules/python.c
@@ -41,7 +41,7 @@
     oldloc = strdup(setlocale(LC_ALL, NULL));
     setlocale(LC_ALL, "");
     for (i = 0; i < argc; i++) {
-        argv_copy[i] = _Py_char2wchar(argv[i]);
+        argv_copy[i] = _Py_char2wchar(argv[i], NULL);
         if (!argv_copy[i])
             return 1;
         argv_copy2[i] = argv_copy[i];