MEDIUM: connection: don't use real send() flags in snd_buf()

This prevents us from passing other useful info and requires the
upper levels to know these flags. Let's use a new flags category
instead : CO_SFL_*. For now, only MSG_MORE has been remapped.
OPTIM: ssl: implement dynamic record size adjustment

By having the stream interface pass the CF_STREAMER flag to the
snd_buf() primitive, we're able to tell the send layer whether
we're sending large chunks or small ones.

We use this information in SSL to adjust the max record dynamically.
This results in small chunks respecting tune.ssl.maxrecord at the
beginning of a transfer or for small transfers, with an automatic
switch to full records if the exchanges last long. This allows the
receiver to parse HTML contents on the fly without having to retrieve
16kB of data, which is even more important with small initcwnd since
the receiver does not need to wait for round trips to start fetching
new objects. However, sending large files still produces large chunks.

For example, with tune.ssl.maxrecord = 2859, we see 5 write(2885)
sent in two segments each and 6 write(16421).

This idea was first proposed on the haproxy mailing list by Ilya Grigorik.