Expose mem_used in show_system_resources

[Yamakaky]

Is your feature request related to a problem?

In Firewall.show_system_resources, only the memory total and memory free metrics are exposed. In unix, total = free + used + buffers, so using memory_free for ram usage computation is inaccurate as it is likely to stay in the high 90s% since unix will use as much buffers as possible

Describe the solution you'd like

It would be nice to expose mem_used (and maybe also mem_buffer?) in this method.

Describe alternatives you've considered

Alternative is doing show system resources manually and parse the output, which is not ideal.

Additional context

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