manpage: vf_vapoursynth: document what happens on seeking

Unfortunately, reloading on seeking causes real problems.
vf_vapoursynth: error out early if the file sub-option is not set

Instead of crashing by possibly passing a NULL pointer to VapourSynth.
vf_vapoursynth: print an error if VapourSynth returns an unknown format

Apparently there is no real format negotiation, so this can actually
happen.
command: add paused-for-cache, total-avsync-change, drop-frame-count properties

This is needed if you want to reimplement the status line in lua

I could only test drop-frame-count because I didn't find an easy way to
trigger paused-for-cache and total-avsync-change

Signed-off-by: wm4 <wm4@nowhere>