[Feature request] Show ZFS ARC statistics

[Caian]

I was discussing this topic on the cockpit github project https://github.com/cockpit-project/cockpit/issues/19482. Apparently the memory statistics show the ARC cache as used memory, so it's always full, which makes it difficult to tell if the system is really running low on RAM.

It would be nice to have a memory readout that distinguishes between used / free memory and ARC.

Since this is specific to ZFS-enabled systems, then maybe this is something that could be implemented here.