runtime(sieve): set fileformat=dos in filetype plugin

References:
https://datatracker.ietf.org/doc/html/rfc5228#section-2.2

closes: #16685

Signed-off-by: David Mandelberg <david@mandelberg.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/sieve.vim b/runtime/ftplugin/sieve.vim
index 3092b5d..8161fe9 100644
--- a/runtime/ftplugin/sieve.vim
+++ b/runtime/ftplugin/sieve.vim
@@ -1,20 +1,19 @@
 " Vim filetype plugin file
 " Language:             Sieve filtering language input file
+" Maintainer:           This runtime file is looking for a new maintainer.
 " Previous Maintainer:  Nikolai Weibull <now@bitwi.se>
-" Latest Revision:      2008-07-09
+" Latest Revision:      2025 Feb 20
 
 if exists("b:did_ftplugin")
   finish
 endif
 let b:did_ftplugin = 1
 
-let s:cpo_save = &cpo
-set cpo&vim
-
-let b:undo_ftplugin = "setl com< cms< fo<"
+let b:undo_ftplugin = "setl com< cms< fo< ff<"
 
 setlocal comments=s1:/*,mb:*,ex:*/,:# commentstring=#\ %s
 setlocal formatoptions-=t formatoptions+=croql
 
-let &cpo = s:cpo_save
-unlet s:cpo_save
+" https://datatracker.ietf.org/doc/html/rfc5228#section-2.2 says
+" "newlines (CRLF, never just CR or LF)"
+setlocal fileformat=dos