Use completion-at-point for mailalias.el

Use completion-at-point for mailalias.el
* lisp/mail/mailalias.el: Use lexical-binding.
(pattern, mailalias-done): Declare dynamic.
(mail-completion-at-point-function): New function, from mail-complete.
(mail-complete): Use it.
(mail-completion-expand): New function.
(mail-get-names): Use it.
(mail-directory, mail-directory-process, mail-directory-stream):
Don't use `pattern' for lexically bound arg.
* lisp/mail/sendmail.el (mail-mode-map): Use completion-at-point.
(mail-mode): Setup mailalias completion here instead.