bug#1586: prefix arg when mouse clicking on header line

[Chong Yidong]

> If a command takes a mouse event and an (optional) prefix arg as its
> arguments, the interactive specifications are
>
>   (interactive "e\nP")
>
> or
>
>   (interactive (list last-input-event current-prefix-arg))
>
> This works fine inside the main part of a window. For a command
> that does the same thing in the header line, these interactive
> specifications do not work.

Thanks for the bug report.  I've committed a fix.