Select the level-0 (`0/<subvolid>`) leaf qgroup row explicitly instead
of taking the last data line, so usage parsing is unambiguous even if an
operator has assigned the subvolume to a higher-level aggregate qgroup.
`btrfs qgroup show -f <path>` already scopes the listing to qgroups
impacting the given path (excluding ancestral qgroups, per
btrfs-qgroup-show(8)); selecting the `0/` leaf among them pins it to the
subvolume's own automatic usage qgroup.
Also: case-insensitive match on the stable "quota not enabled" error
fragment (wording varies across btrfs-progs versions), `# Errors` doc
sections on the three public priv_client quota functions, and precise
doc comments on the `-f` flag semantics.
681e993626