The I/O code can receive incremental file-list chunks during deletion,
and their OPT_EXTRA fields would get corrupted when file_extra_cnt is
incremented.
Instead of temporarily enabling uid_ndx to find out whether the user
owns a file, have make_file() set a flag for that purpose.
Applied with a few minor tweaks by Wayne. Fixes bug 7936.
- Backups do not interfere with an atomic update (when possible).
- Backing up a file will remove a directory that is in the way
and visa versa.
- Unify the backup-dir and non-backup-dir code in backup.c.
- Improved the backup tests a little bit.
