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

scripts/dns-nsec3-enum.nse

@@ -2,7 +2,6 @@ local stdnse = require "stdnse"
 local shortport = require "shortport"
 local dns = require "dns"
 local base32 = require "base32"
-local openssl = require "openssl"
 local msrpc = require "msrpc" -- just for random string generation
 local math = require "math"
 local bin = require "bin"
@@ -10,6 +9,8 @@ local nmap = require "nmap"
 local string = require "string"
 local table = require "table"
 
+local openssl = stdnse.silent_require "openssl"
+
 description = [[
 Tries to enumerate domain names from the DNS server that supports DNSSEC
 NSEC3 records.

scripts/http-userdir-enum.nse

@@ -3,7 +3,6 @@ local bin = require "bin"
 local datafiles = require "datafiles"
 local http = require "http"
 local nmap = require "nmap"
-local openssl = require "openssl"
 local os = require "os"
 local shortport = require "shortport"
 local stdnse = require "stdnse"
@@ -158,7 +157,8 @@ end
 
 function randomstring()
   local rnd, s, l, _
-  if pcall(require, "openssl") then
+  local status, openssl = pcall(require, "openssl")
+  if status then
     rnd = openssl.rand_pseudo_bytes
   end
   s = rnd and rnd(8) or tostring( os.time() )

scripts/http-virustotal.nse

@@ -1,8 +1,8 @@
 local http = require "http"
 local io = require "io"
 local json = require "json"
-local openssl = require "openssl"
 local stdnse = require "stdnse"
+local openssl = stdnse.silent_require "openssl"
 local tab = require "tab"
 local table = require "table"
 

scripts/smb-ls.nse

@@ -3,7 +3,7 @@ local smb    = require 'smb'
 local stdnse = require 'stdnse'
 local tab    = require 'tab'
 local table = require "table"
-local openssl= require 'openssl'
+local openssl= stdnse.silent_require 'openssl'
 
 description = [[
 Attempts to retrieve useful information about files shared on SMB volumes.

scripts/svn-brute.nse

@@ -1,9 +1,9 @@
 local brute = require "brute"
 local creds = require "creds"
 local nmap = require "nmap"
-local openssl = require "openssl"
 local shortport = require "shortport"
 local stdnse = require "stdnse"
+local openssl = stdnse.silent_require "openssl"
 
 description = [[
 Performs brute force password auditing against Subversion source code control servers.