bug#25439: 24.5; `forward-*' cmds: numeric ARG should always be optional

[Eli Zaretskii]

> Date: Fri, 13 Jan 2017 08:49:03 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> The numeric argument to most `forward-*' commands is optional,
> defaulting to 1.  This should be the case in general.  These three
> commands require their ARG, but they should not.  It should be
> optional and default to 1.
> 
>  `forward-button'
>  `forward-symbol'
>  `forward-whitespace'

The last two are okay in interactive usage.