Don't forcibly turn on the mode upon load.

Don't forcibly turn on the mode upon load.
(icomplete-mode): Use define-minor-mode.
(icomplete-eoinput): Default to nil.
(icomplete-minibuffer-setup): Remove autoload.
(icomplete-tidy): Simplify.
(icomplete-exhibit): Use buffer-undo-list to determine if we're still
in the initial state or if the user has modified the field.
Fix handling of icomplete-max-delay-chars.
Remove code that handles the oddball case where
minibuffer-completion-table is an integer.
Wrap icomplete-completions in while-no-input in case building
completions takes more time than expected.
(icomplete-completions): Simplify.