Set the math.randomseed value in nse_main.lua on behalf of scripts.

Since Lua uses the C rand and srand functions, which have a static
seed for the entire program, we don't want scripts doing this themselves.

scripts/nping-brute.nse

@@ -180,7 +180,6 @@ Driver =
 }
 
 action = function(host, port)
-	math.randomseed(nmap.clock())
 	local engine = brute.Engine:new(Driver, host, port)
 	engine.options.firstonly = true
 	engine.options:setOption("passonly", true)