Avoid changing file_extra_cnt during deletion.

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.