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/ssh2-enum-algos.nse

@@ -1,6 +1,7 @@
 local nmap = require "nmap"
 local shortport = require "shortport"
 local stdnse = require "stdnse"
+local string = require "string"
 local table = require "table"
 
 local openssl = stdnse.silent_require "openssl"