Make MH-E use completion-at-point

Make MH-E use completion-at-point
* lisp/mh-e/mh-letter.el (mh-letter-completion-at-point): New function,
extracted from mh-letter-complete
(mh-letter-mode, mh-letter-complete, mh-letter-complete-or-space): Use it.
(mh-complete-word): Only use the common-substring arg when it works.
(mh-folder-expand-at-point):
* lisp/mh-e/mh-alias.el (mh-alias-letter-expand-alias): Return data suitable
for completion-at-point-functions.
* lisp/mh-e/mh-utils.el (mh-folder-completion-function): Make it work like
file-name completion, so partial-completion can do its job.
* lisp/minibuffer.el (completion-at-point, completion-help-at-point):
Don't presume that a given completion-at-point-function will always
use the same calling convention.