Contrib: vim syntax adjusted to save cpoptions (ticket #2276).

Line continuation as used in the syntax file might be broken if "compatible"
is set or "C" is added to cpoptions.  Fix is to set the "cpoptions" option
to vim default value at script start and restore it later, see
":help use-cpo-save".

contrib/vim/syntax/nginx.vim

@@ -5,6 +5,9 @@ if exists("b:current_syntax")
   finish
 end
 
+let s:save_cpo = &cpo
+set cpo&vim
+
 " general syntax
 
 if has("patch-7.4.1142")
@@ -2485,4 +2488,7 @@ hi def link ngxDirectiveThirdPartyDeprecated Error
 hi def link ngxListenOptions Keyword
 hi def link ngxListenOptionsDeprecated Error
 
+let &cpo = s:save_cpo
+unlet s:save_cpo
+
 let b:current_syntax = "nginx"