runtime: set 'cpoptions' for line-continuation in various runtime files

closes: #17121

Signed-off-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/nroff.vim b/runtime/ftplugin/nroff.vim
index 7d3d2a5..1c6d4eb 100644
--- a/runtime/ftplugin/nroff.vim
+++ b/runtime/ftplugin/nroff.vim
@@ -7,12 +7,16 @@
 " Last Changes:
 "	2024 May 24 by Riley Bruins <ribru17@gmail.com> ('commentstring' #14843)
 "	2025 Feb 12 by Wu, Zhenyu <wuzhenyu@ustc.edu> (matchit configuration #16619)
+"	2025 Apr 16 by Eisuke Kawashima (cpoptions #17121)
 
 if exists("b:did_ftplugin")
   finish
 endif
 let b:did_ftplugin = 1
 
+let s:cpo_save = &cpo
+set cpo&vim
+
 setlocal commentstring=.\\\"\ %s
 setlocal comments=:.\\\"
 setlocal sections+=Sh
@@ -30,3 +34,6 @@
         \ . ',^\.\s*FS\>:^\.\s*FE\>'
   let b:undo_ftplugin .= "| unlet b:match_words"
 endif
+
+let &cpo = s:cpo_save
+unlet s:cpo_save