(message-generate-new-buffers): Change the meaning of the nil value;
add `standard' to the choices; treat t as `unique'; improve doc string.
(gnus-select-frame-set-input-focus): Autoload.
(message-buffer-name): Search for the existing message buffer if
message-generate-new-buffers is nil or `standard';
treat the value t of message-generate-new-buffers as `unique'.
(message-pop-to-buffer): Raise the frame already displaying the message buffer;
clear the echo area after querying.
(message-setup): Pass the `continue' argument to compose-mail.
(message-mail): Prefer `switch-function' if it is given;
search for the existing message buffer if the `continue' argument is non-nil;
pass continue and switch-function arguments to compose-mail by way of
message-setup.
(message-mail-other-window): Adjust argument of message-setup.
(message-mail-other-frame): Ditto.