[PATCH setup] Don't write garbage into the "extrakeys" user setting

[Ken Brown]

On 11/27/2017 4:35 PM, Ken Brown wrote:
> The ExtraKeysSetting destructor called UserSettings::set() on a string
> that was terminated by LF instead of NUL.  This led to garbage at the
> end of the "extrakeys" setting that was written into setup.rc.  Fix
> this by replacing the final LF by NUL before calling set().

Disregard this for now.  I've just discovered some further issues 
involving "extrakeys", and I'd like to fix them all at once.

Ken