Default custom group

[Richard Stallman]

While cleaning up etc/NEWS I was reminded that the feature
if the default group in defcustom is new in this release.
I have never been happy with this default, and the best time
to change things is now, before we make a release with it.

So I just made the byte compiler warn whenever a custom definition
fails to specify a group.

At some time in the future we should make them get errors at
run time if they don't specify a group, but not right away.

This default feature was not documented in the Lisp Manual,
so I had no change to make there.