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.

[Soapsuds]

I also ran into the problem with files getting overwritten due to timestamp information being missing on some of my clips. This patch resolves the issue and still allows me to use timestamp mode. Thanks!