* lisp/minibuffer.el (set-message-functions): New user option.

* lisp/minibuffer.el (set-message-functions): New user option.

(set-message-function): Change the default from
'set-minibuffer-message' to 'set-message-functions'.
'set-minibuffer-message' is set as the default value
of the user option 'set-message-functions'.
(set-message-functions): New function.
(inhibit-message-regexps): New customizable variable (bug#52314).
(inhibit-message): New function.
(multi-message-timeout, multi-message-max): New defcustoms.
(multi-message-separator, multi-message-list): New variables.
(set-multi-message): New function.