A Proxmox host's root filesystem stood at 94 % with about 400 MB free for a
week; a kernel upgrade failed on it with "No space left on device" and left
dpkg halfway, and nothing had said the disk was full (NetOrk/netork#862).
HOST_STATUS_COMMAND now also runs POSIX `df -Pk / /boot /var`, without root.
get_host_status() returns `filesystems`: each filesystem once (a directory
that is not a mount of its own is the filesystem it lives on), with its
mountpoint, total, used and available bytes, or None when df said nothing.
`available` is what df says an ordinary user can still write, which on ext4
is less than total minus used. How full is too full is the caller's decision.
Version 4.4.0.
christianmanivong