Make a few more tests pass with the new I/O library.

Fix the truncate() semantics -- it should not affect the current position.
Switch wave.py/chunk.py to struct.unpack_from() to support bytes.
Don't use writelines() on binary files (test_fileinput.py).

Lib/test/test_fileinput.py

@@ -18,7 +18,8 @@ from fileinput import FileInput, hook_encoded
 def writeTmp(i, lines, mode='w'):  # opening in text mode is the default
     name = TESTFN + str(i)
     f = open(name, mode)
-    f.writelines(lines)
+    for line in lines:
+        f.write(line)
     f.close()
     return name