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.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "napalm-device-types"
-version = "4.3.1"
+version = "4.4.0"
 description = "Abstract device-type base classes for NAPALM drivers"
 readme = "README.md"
 requires-python = ">=3.10"