v6.8.1

• mkfs: fix writing on zoned device when block-group-tree is selected
• tune: fix writing on zoned device with option --convert-to-block-group-tree
• check:
  • more progress and error messages
  • unify handling of unknown command line options with other commands
• subvolume delete: remove options --delete-qgroup and --no-delete-qgroup (added in 6.6.3), qgroup deletion does not always work due to delayed background processing of subvolume or set value in sysfs:ggroup/drop_subtree_threshold
• other:
  • misc refactoring
  • error handling fixes reported by gcc -fanalyzer
  • documentation updates
  • new and updated tests