Add support for --open-mode u (read and write mode)

[thiell]

This patch slightly changes the logic of --open-mode to select files that are open for:

• read (r)
• write (w)
• read and write (u)

This makes -o filtering more flexible and possible with copy tools that open the source as read+write for example.

The character "u" is also used by lsof for those files.

[Xfennec]

Nice idea, thanks!