[21.2]: M-x customize-group offers non-groups for customisation

["Marshall, Simon" <simon.marshall@misys.com>]

If you do M-x customize-group RET then you can enter, e.g.,
auto-compression-mode and a customisation buffer is offered up.  Problem
is, auto-compression-mode isn't a group and so the customisation buffer
is empty/confusing.

The cause is in this in customize-group:

(completing-read "Customize group: (default emacs) "
     obarray
     (lambda (symbol)
       (or (get symbol 'custom-loads)
           (get symbol 'custom-group)))
     t)

where auto-compression-mode has a non-nil custom-loads property (its
custom-group property is nil).

How hard is it to make custom only offer genuine groups?  Presumably too
hard.

Perhaps something meaningful can be done after customize-group does the
custom-load-symbol, e.g., say, "oh, it's not a group, but here's a
customisation buffer containing the list of groups to which it belongs"
or "oh, it's not a group, it's an X and here's the appropriate
customisation buffer to customise it".

Simon.