custom-mode should not be interactive

[Magnus Henoch]

I noticed that custom-mode is an interactive function now.  I think it
shouldn't be, for two reasons:

1. It doesn't do anything useful when called interactively.
2. It hinders tab completion, making M-x customize-variable etc more
   difficult to type.

I'm not sure how to fix it though, as custom-mode is defined through
define-derived-mode, which always makes the mode function interactive.
One way is to define a new keyword argument for define-derived-mode.
What do you think?

Magnus