fix: Don't overwrite files in timecode/timestamp/timsys mode

[mfep]

• Sometimes subsequently recorded clips are written to the same path, erasing the previously written file.
  • This happens, e.g. when the dvgrab operates in --timestamp mode, and some recordings on the camera don't have timestamp information.
• This patch addresses this problem by checking if the output file exists, and if so, the filename is postfixed with a numeric counter.
  • This is similar to the default filename composition logic.