Use stdnse.silent_require where needed.

This is required for scripts that require "openssl", but also
transitively for script that require a library that requires "openssl".

scripts/broadcast-ping.nse

@@ -49,10 +49,10 @@ categories = {"discovery","safe","broadcast"}
 require "nmap"
 require "stdnse"
 require "packet"
-require "openssl"
 require "bin"
 require "tab"
 require "target"
+stdnse.silent_require("openssl")
 
 prerule = function()
 	if not nmap.is_privileged() then