Make gnus/message.el work correctly under lexical binding

* lisp/gnus/message.el (message-send-mail): Rename `arg' to
`_' to shut up the byte compiler.
(sha1-maximum-internal-length, smtpmail-smtp-server)
(smtpmail-smtp-service): add missing defvars

lisp/gnus/message.el

@@ -4481,7 +4481,7 @@ This function could be useful in `message-setup-hook'."
 
 (declare-function hashcash-wait-async "hashcash" (&optional buffer))
 
-(defun message-send-mail (&optional arg)
+(defun message-send-mail (&optional _)
   (require 'mail-utils)
   (let* ((tembuf (message-generate-new-buffer-clone-locals " message temp"))
 	 (case-fold-search nil)
@@ -4638,6 +4638,8 @@ If you always want Gnus to send messages in one piece, set
     (push 'mail message-sent-message-via)))
 
 (defvar sendmail-program)
+(defvar smtpmail-smtp-server)
+(defvar smtpmail-smtp-service)
 (defvar smtpmail-smtp-user)
 
 (defun message-multi-smtp-send-mail ()
@@ -4817,6 +4819,8 @@ command evaluates `message-send-mail-hook' just before sending a message."
   (run-hooks 'message-send-mail-hook)
   (mailclient-send-it))
 
+(defvar sha1-maximum-internal-length)
+
 (defun message-canlock-generate ()
   "Return a string that is non-trivial to guess.
 Do not use this for anything important, it is cryptographically weak."