bug#13391: dd silently ignores lseek error

[Philip Rowlands]

> From ea524ab7388bb35e591dcdb0fc7f7989d61143ae Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?P=C3=A1draig=20Brady?= <address@hidden>
> Date: Wed, 9 Jan 2013 00:42:38 +0000
> Subject: [PATCH] dd: add [io]flag=seekable to verify file support for lseek
>
> * src/dd.c: Add the new O_SEEKABLE flag.
> (main): Verify leek() works if O_SEEKABLE is set.

leek?

> (usage): Describe the new flag.
> * tests/dd/misc.sh: Augment the test for the new options.
> * doc/coreutils.texi (dd invocation): Describe the new option.
> * cfg.mk (sc_dd_O_FLAGS): Add O_SEEKABLE to the list of private
> flags with a a single underscore.
> * NEWS: Mention the new feature.