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/pgsql.lua

@@ -14,8 +14,8 @@
 
 local bin = require "bin"
 local nmap = require "nmap"
-local openssl = require "openssl"
 local stdnse = require "stdnse"
+local openssl = stdnse.silent_require "openssl"
 local string = require "string"
 local table = require "table"
 _ENV = stdnse.module("pgsql", stdnse.seeall)