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/sip-brute.nse

@@ -73,7 +73,6 @@ Driver = {
 -- was successful which makes it impossible to tell successfull logins
 -- from non-existing accounts apart.
 local function checkBadUser(host, port)
-	math.randomseed( os.time() )
 	local user = "baduser-" .. math.random(10000)
 	local pass = "badpass-" .. math.random(10000)
 	local helper = sip.Helper:new(host, port, { expires = 0 })
@@ -103,4 +102,4 @@ action = function(host, port)
 	local engine = brute.Engine:new(Driver, host, port)
 	local status, result = engine:start()
 	return result
-end
+end