filelock: StrictSoftFileLock creates its claim directories without the lock mode, so another local user with a write bit on them can end a live hold

StrictSoftFileLock and AsyncStrictSoftFileLock apply the caller's mode to the sentinel the lock file at and to both claim files the files whose presence says a process wants or holds the lock. They create the two directories that hold the claims, .filelock and .filelock/claims, with a bare mkdir...