add new stdio extension functions to make gnulib happy

this is mildly ugly, but less ugly than gnulib trying to poke at the
definition of the FILE structure...

src/stdio/ext2.c

+#include "stdio_impl.h"
+
+size_t __freadahead(FILE *f)
+{
+	return f->rend - f->rpos;
+}
+
+const char *__freadptr(FILE *f, size_t *sizep)
+{
+	size_t size = f->rend - f->rpos;
+	if (!size) return 0;
+	*sizep = size;
+	return f->rpos;
+}
+
+void __freadptrinc(FILE *f, size_t inc)
+{
+	f->rpos += inc;
+}
+
+void __fseterr(FILE *f)
+{
+	f->flags |= F_ERR;
+}