SSL overhaul fixing OpenSSL related problems when SSL has not been compiled in

* replace require function calls with stndse.silent_require
* fixed a bug in nse_main that would fail creating scripts.db when a script
  fails to load
* reworked some code to provide limited functionality even though SSL is not
  present

nselib/ssh1.lua

@@ -10,8 +10,8 @@ local bin = require "bin"
 local bit = require "bit"
 local math = require "math"
 local nmap = require "nmap"
-local openssl = require "openssl"
 local stdnse = require "stdnse"
+local openssl = stdnse.silent_require "openssl"
 _ENV = stdnse.module("ssh1", stdnse.seeall)
 
 --- Retrieve the size of the packet that is being received