When we look for a new trail file there might be a race between find trail
file name and opening it. This race was not properly handled, because we were
copying new name before checking for openat(2) error and when we were trying
again we were starting with the next trail file. This could result in skipping
distribution of such a trail file.
Fix this problem by checking for ENOENT first (only for .not_terminated files)
and then updating (or not) tr_filename before restarting the search.
Reported by: peter
Approved by: re (kib)