Issue #6236, #6348: Fix various failures in the io module under AIX
and other platforms, when using a non-gcc compiler. Patch by egreen.

In addition, I made explicit the signedness of all bitfields in the
IO library.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 3b627f2..2f99a60 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -190,9 +190,9 @@
     PyObject_HEAD
     PyObject *decoder;
     PyObject *errors;
-    int pendingcr:1;
-    int translate:1;
-    unsigned int seennl:3;
+    signed int pendingcr: 1;
+    signed int translate: 1;
+    unsigned int seennl: 3;
 } nldecoder_object;
 
 static int