DEV: h2: support reading frame payload from a file

DEV: h2: support reading frame payload from a file

Now we can build a series of data frames by reading from a file and
chunking it into frames of requested length. It's mostly useful for
data frames (e.g. post). One way to announce these upfront is to
capture the output of curl use without content-length:

  $ nc -lp4446 > post-h2-nocl.bin
  $ curl -v --http2-prior-knowledge http://127.0.0.1:4446/url -H "content-length:" -d @/dev/null

Then just change the 5th byte from the end from 1 to 0 to remove the
end-of-stream bit, it will allow to chain a file, then to send an
empty DATA frame with ES set :

  $ (dev/h2/mkhdr.sh -i 1 -t data -d CHANGELOG;
     dev/h2/mkhdr.sh -i 1 -t data -l 0 -f es) > h2-data-changelog.bin

Then post that to the server:
  $ cat post-h2-nocl.bin h2-data-changelog.bin | nc 0 4446