Add syntax files from upstream vim repository

syntax/dns.vim

@@ -0,0 +1,9 @@
+if !exists('g:polyglot_disabled') || index(g:polyglot_disabled, 'vim') == -1
+  
+" Vim syntax file
+" Language:     DNS/BIND Zone File
+
+" This has been replaced by the bindzone syntax
+:runtime! syntax/bindzone.vim
+
+endif