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.

nselib/rpc.lua

@@ -120,8 +120,6 @@ RPC_version = {
     ["mountd"] = { min=1, max=3 },
 }
 
-math.randomseed( os.time() )
-
 -- Low-level communication class
 Comm = {