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/fileio.c b/Modules/_io/fileio.c
index 989c866..e98de9e 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -45,10 +45,10 @@
 typedef struct {
 	PyObject_HEAD
 	int fd;
-	unsigned readable : 1;
-	unsigned writable : 1;
-	int seekable : 2; /* -1 means unknown */
-	int closefd : 1;
+	unsigned int readable : 1;
+	unsigned int writable : 1;
+	signed int seekable : 2; /* -1 means unknown */
+	unsigned int closefd : 1;
 	PyObject *weakreflist;
 	PyObject *dict;
 } fileio;