[Q GNUS] Set defcustom defaults to nil to avoid byte-compile problem

Mike Kupfer writes:

 > I'm a little unclear on whether we are still using the convention
 > of tagging changes like this with
 > 
 >   ;; XEmacs change  
 > 
 > Can someone clarify that for me?

It's informal, not written anywhere as far as I know.  It basically
means "don't sync blindly because XEmacs does things differently."
Usually it should be accompanied with a brief explanation of why.

If you don't want it in Gnus, just say so.  It's most useful in the
core or packages maintained by the XEmacs Dev Team since many
different people may sync code.

N.B. Perhaps Mike or someobdy remembers differently.  If so, they're
probably right. :-)