* lisp/gnus/message.el (message-expand-name-standard-ui): New option

* lisp/gnus/message.el (message-expand-name-standard-ui): New option

(message--old-style-completion-functions): New var.
(message-completion-function): Allow functions on
`message-completion-alist` to follow the capf protocol.
(message-completion-alist): Adjust docstring accordingly.
Simplify regexps and make them apply more liberally.
(message-expand-group): Use the capf protocol.
(completion-category-defaults): Use 'substring' completion style by
default for email addresses.
(message--bbdb-query-with-words, message--name-table): New functions.
(message-expand-name): Use them to obey `message-expand-name-standard-ui`.