even better ffmetadata handling

- -f flag now sets the search directory for where the track.ffmetadata
  files are stored. The default is the input directory.

- *.ffmetadata is never an acceptable ffmpeg input format except for
  inputting metadata. Skip it when iterating the directory. This
  obsoletes the need for -e flag when using ffmetadata.

- Autodetect the ffmetadata file. If it exists, use it. If not, ignore
  the fact and move along happily.

- Do not recompute track title from ffmetadata file for use with
  -metadata option. If that happens and the field has an '=' in it,
  only the final field is used. Not good.
force no leading zeros in embedded track number

If there are leading zeros in the track number, do not include them
when writing the metadata. Leading zeros are for shell sorting. When
reading metadata from a file, more control can be used then.