FAT performance improvement. In large files, seeking to a position fromt he beginning of the file can be very time consuming.  ftell does lssek(fd, 0, SET_CURR).  In that case, that is wasted time since we are going to seek to the same position.  This fix short-circutes fat_seek() in all cases where we attempt to seek to curren position.  Suggested by Nate Weibley