Linux VM page cache (#1270): QEMU VM memory now falls back to Proxmox
RRD's memavailable metric (which excludes reclaimable page cache) when
the qemu-guest-agent doesn't provide MemInfo.Available. Previously the
fallback was detailedStatus.Mem (total - MemFree), inflating usage to
80%+ on VMs with normal Linux page cache. Mirrors the existing LXC
rrd-memavailable path.
FreeBSD ZFS ARC (#1264, #1051): The host agent now reads
kstat.zfs.misc.arcstats.size via SysctlRaw on FreeBSD and subtracts
the ARC size from reported memory usage. ZFS ARC is reclaimable under
memory pressure (like Linux SReclaimable) but gopsutil counts it as
wired/non-reclaimable, causing false 90%+ memory alerts on TrueNAS
and FreeBSD hosts. Build-tagged so it compiles cleanly on all platforms.
Fixes#1270Fixes#1264Fixes#1051
(cherry picked from commit 94502f83ff9ffc6da28aaadc946a2f7d8b4e9bac)