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

[Drew Adams]

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'

In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'