feat: report how full /, /boot and /var are in the host status

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.

README.md

@@ -84,7 +84,8 @@ a reboot to finish an update (`/var/run/reboot-required`, `needs-restarting -r`,
 kernel of the running flavour installed), whether it patches itself (unattended-upgrades,
 dnf-automatic) and when that last succeeded (apt's `upgrade-stamp`), and whether dpkg was
 left halfway (a journal in `/var/lib/dpkg/updates` or a half-configured package; unknown
-while a package manager is running). `package_updates` holds the shared apt and dnf parsers: apt's suites become
+while a package manager is running), and how full `/`, `/boot` and `/var` are (`df -Pk`,
+each filesystem once, in bytes; how full is too full is the caller's decision). `package_updates` holds the shared apt and dnf parsers: apt's suites become
 an update's `origin`, a `-security` suite makes it a security update, and dnf's security
 advisories do the same.