Make Checkmk Memory/RAM monitoring zfs arc aware

hello,
checkmk memory checking does not honour zfs arc special case and counts zfs arc as used memory instead of buffers/caches. this triggers unnecessary warning for low memory, where it doesn't exist.

this is due to zfs arc being a special case ( https://github.com/openzfs/zfs/issues/10251 ) , which needs special handling

please consider making checkmk server + agent memory checking zfs arc aware, i.e. with checking memory values, also check if zfs is active and using arc. that should not be hard to do , see https://bugzilla.proxmox.com/show_bug.cgi?id=4482 or https://bugzilla.proxmox.com/show_bug.cgi?id=4482 for example

also see
https://forum.checkmk.com/t/how-to-monitor-memory-on-zfs-systems/53374