bug#11301: 24.1.50; `define-minor-mode' should set `custom-mode-group' according to :group

[Drew Adams]

OK, so I looked a bit further.  For major or minor mode `foo-mode':

`custom-group-of-mode' checks for property `custom-mode-group' on the symbol
`foo-mode'.

Lacking that, it checks for property `custom-group' on the same symbol.

Lacking that, it checks for `custom-group' on symbol `foo'.

So if we look at `icomplete-mode', which is defined using
`define-minor-mode' specifying :group as `icomplete':
(custom-group-of-mode 'icomplete-mode) returns non-nil,
because symbol `icomplete' gets property `custom-group'.
Where does that come from?  

It seems to come from the :group.  But the :group value is not applied as
property `custom-group' to the _mode_ symbol.  It is applied only to the :group
value itself.  E.g., with :group 'Foo, property `custom-group' is applied to the
symbol `Foo' but not (also) to the mode name `foo-mode' or even `foo'.

So if you use as the :group value something that is not _identical_ to the
mode-function symbol (or that minus `-mode'), then `custom-group-of-mode' does
not work.  Its attempt to find the mode's group is too limited.  If you use
:group 'Foo or :group 'Foos, you are out of luck.  You must use :group 'foo for
`foo-mode' to be recognized by `custom-group-of-mode'.  (And no such limitations
are documented.)

Please DTRT: pick up whatever :group specifies as the group and include it in
the value of property `custom-group' for the mode name.