Fixed global assignments with nse_check_globals

All fixes made by hand. A couple real bugs/errors fixed, due to
copy-paste of code from other scripts without changing variable names.

scripts/http-vhosts.nse

@@ -1,5 +1,6 @@
 local coroutine = require "coroutine"
 local http = require "http"
+local io = require "io"
 local nmap = require "nmap"
 local shortport = require "shortport"
 local stdnse = require "stdnse"