Change internals of FILE: reuse the unget buffer field to access an

extended attribute data structure (pimpl idiom). Idea taken from citrus.
Much discussion with deraadt@, otto@, millert@...
This is the least disruptive way to extend FILE, since its size can't really
change without this being a flag day. So the size doesn't change.
Actual additions to the structure will come in separate steps, since this
change is nasty enough on its own.
Tests by otto@ and others, careful reading of code by otto@ and millert@.
This is definitely a major bump, and has been checked to not impact a
full ports build.

src/include/stdio.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: stdio.h,v 1.30 2004/06/07 21:11:23 marc Exp $	*/
+/*	$OpenBSD: stdio.h,v 1.31 2005/04/30 09:25:17 espie Exp $	*/
 /*	$NetBSD: stdio.h,v 1.18 1996/04/25 18:29:21 jtc Exp $	*/
 
 /*-
@@ -119,8 +119,9 @@ typedef	struct __sFILE {
 	fpos_t	(*_seek)(void *, fpos_t, int);
 	int	(*_write)(void *, const char *, int);
 
-	/* separate buffer for long sequences of ungetc() */
-	struct	__sbuf _ub;	/* ungetc buffer */
+	/* extension data, to avoid further ABI breakage */
+	struct	__sbuf _ext;
+	/* data for long sequences of ungetc() */
 	unsigned char *_up;	/* saved _p when _p is doing ungetc data */
 	int	_ur;		/* saved _r when _r is counting ungetc data */