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/afp.lua

@@ -114,18 +114,13 @@
 local bin = require "bin"
 local bit = require "bit"
 local nmap = require "nmap"
-local openssl = require "openssl"
 local os = require "os"
 local stdnse = require "stdnse"
 local string = require "string"
 local table = require "table"
 _ENV = stdnse.module("afp", stdnse.seeall);
 
-local HAVE_SSL = false
-
-if pcall(require,'openssl') then
-  HAVE_SSL = true
-end
+local HAVE_SSL, openssl = pcall(require,'openssl')
 
 -- Table of valid REQUESTs
 local REQUEST = {