Fix borderless for max layout

For each window you open on a screen in max layout, the problem becomes more evident: The arrange signal handler is called for each window (not only the one in the front), while in its loop

    elseif #clients == 1 or layout == "max" then
        clients[1].border_width = 0
        awful.client.moveresize(0, 0, 2, 2, clients[1])
    else

client[1] from the table is resized each time. The more windows you stack, this leads to one slightly oversized window, while the others stayed the same.

This also lead to some new windows strangely getting placed on the wrong taskbar in my multi-monitor setup (Windows opens on the right screen but is in the left taskbar). This seems to have gone away, too.