(completion-in-region): Pop down *Completions* automatically.

(completion-in-region): Pop down *Completions* automatically.
* lisp/minibuffer.el (completion-table-dynamic): Optimize `boundaries'.
(completion-in-region-mode): New minor mode.
(completion-in-region): Use it.
(completion-in-region--data, completion-in-region-mode-map): New vars.
(completion-in-region--postch): New function.
(completion--capf-misbehave-funs, completion--capf-safe-funs): New vars.
(completion--capf-wrapper): New function.
(completion-at-point): Use it to track well-behavedness of hook functions.
(completion-help-at-point): New command.