Add to sendmail.el a new command mail-add-attachment.

Add to sendmail.el a new command mail-add-attachment.

 etc/NEWS: Document the new command and the alias for mail-insert-file.
 lisp/mail/sendmail.el (mml-to-mime, mml-attach-file)
 (mm-default-file-encoding): Remove autoload forms, they are
 replaced with autoload cookies in mml.el and mm-encode.el.
 (mail-add-attachment): New command.
 (mail-mode-map): Add a menu-bar item for mail-add-attachment.
 (mail-mode): Mention mail-insert-file and mail-add-attachment in
 the doc string.
 (mml-to-mime, mml-attach-file, mm-default-file-encoding): Declare.
 lisp/gnus/mm-encode.el (mm-default-file-encoding): Add autoload cookie.
 lisp/gnus/mml.el (mml-to-mime, mml-attach-file): Add autoload cookies.
 doc/emacs/sending.texi (Mail Misc): Document mail-add-attachment.