doc: Clarify the minimum Libav version

And add some links.
doc: Clarify NUT usage
modes: Refactor modes listing in a stand alone file
doc: Drop superseded -I option from the README
bmdcapture: Add a missing break
bmdplay: Cleanly exit if audio or video streams are missing

Currently bmdplay does not support audio-only or video-only
input.
bmdcapture: Factorize the data stream writer

Will be reused for different kind of metadata.
bmdcapture: Add a wallclock option

Useful to measure the pipeline delay.
doc: Suggest to use vsync passthrough

The option makes sure no timestamp mangling happens.