new switch added to useradd command, --btrfs-subvolume-home. When
specified *and* the filesystem is detected as btrfs, it will create a
subvolume for user's home instead of a plain directory. This is done via
`btrfs subvolume` command. Specifying the new switch while trying to
create home on non-btrfs will result in an error.
userdel -r will handle and remove this subvolume transparently via
`btrfs subvolume` command. Previosuly this failed as you can't rmdir a
subvolume.
usermod, when moving user's home across devices, will detect if the home
is a subvolume and issue an error messages instead of copying it. Moving
user's home (as subvolume) on same btrfs works transparently.
c1d36a8acb