fbff: add periodic syncs

fbff: add periodic syncs

Now -s argument can accept a number; -s100 means sync every hundred frames
(note that there is no space between -s and 100).  This is specially
helpful for files that get out of sync without -s, and -s results in
choppy playback.  The -s without any number is equivalent to -s1 and
means sync every frame.

So to get the best playback I suggest:
* try playback with -s; if out of sync, you need to use -/+ commands
  to set avdiff.  The best guide for avdiff is the amount reported in
  'i' command just when the playback starts without -s.
* try playback with -s100 and just after starting the playback press
  'a' command to record the avdiff