New command :VimwikiCheckLinks

while we are at it, refactor the code to search through files

syntax/vimwiki.vim

@@ -13,8 +13,9 @@ endif
 "TODO do nothing if ...? (?)
 let g:starttime = reltime()  " start the clock
 if VimwikiGet('maxhi')
-  let b:existing_wikifiles = vimwiki#base#get_links('*'.VimwikiGet('ext'))
-  let b:existing_wikidirs  = vimwiki#base#get_links('*/')
+  let b:existing_wikifiles = vimwiki#base#get_wikilinks(g:vimwiki_current_idx)
+  let b:existing_wikidirs  =
+        \ vimwiki#base#get_wiki_directories(g:vimwiki_current_idx)
 endif
 let s:timescans = vimwiki#u#time(g:starttime)  "XXX
   "let b:xxx = 1