dell: fix save_config for Dell Force10 / OS9 devices

On a Dell S4048-ON switch running OS9 version 9.14(2.5), the current
save_config() implementation does not work:

    sw#copy running-configuration startup-configuration
    % Error: The source file does not exist.

There are two ways to save on OS9: either "copy running-config
startup-config" (but it requires a confirmation), or "write memory".
We use the latter to avoid dealing with the confirmation prompt.

Also tested on a Dell S4810 running Dell OS 8.3.10.1, it has the same
behaviour.

Unfortunately I don't have access to a device with the original Force10
OS, but according to old manuals found online, "write memory" seems to
also be supported.