First step towards using standard completion in comint.

First step towards using standard completion in comint.
* lisp/minibuffer.el (completion--flush-all-sorted-completions):
Remove itself from hook.
(completion-at-point): Let the functions perform the completion
immediately and return nil or t.
* lisp/comint.el (comint-dynamic-complete-functions): Now identical to
completion-at-point-functions.
(comint-dynamic-list-input-ring): Remove unused var `index'.
(comint--match-partial-filename, comint--unquote&expand-filename):
New funs, split from comint-match-partial-filename.
(comint-dynamic-complete): Use completion-at-point.
(comint-dynamic-complete-filename): Use comint--match-partial-filename.