#4 f50 uses different filename offset to f60

[Lewis Jardine]

the f60 stores its filenames starting at [i*256]+21 .
However, my f50 appears to store them at [i*256]+18 .

The first three characters of the path are missing when
using an unmodified mputil, however this is not
obvious, as (with mp3/mpms) the characters are missing
from the prefixed path which is removed by f60_strip_path.

Data files uploaded by the mpman software, however,
have no path prefix, so the missing characters are obvious.