If SPDY or HTTP/2 ustream is used and HTTP/2 downstream is used, only
call {spdylay,nghttp2}_resume_data when complete DATA frame was read
in backend to avoid to transmit too small DATA frame to the upstream.

.. instead of nghttp2_stream_attach_data() internal API

To advertise alternative serive, at least --altsvc-port and
--altsvc-protocol-id must be specified.

Combine deferred_data and data into data_item and merge deferred_flags
into flags.

This is preliminary change for upcoming HPACK updates.  The flags are
used to determine the name/value pair is indexable or not.

We do -+1 on serialization and deserialization since the field is 1
byte.  This change also parameterized range so that we can change it
easily.

Non printable ascii is printed as ".".

This commit also fixes defaltehd always reports output length is 0.

This identifier string is used if HTTP/2 is used over cleartext TCP.

NGHTTP2_CLIENT_CONNECTION_PREFACE has the same content with
NGHTTP2_CLIENT_CONNECTION_HEADER, which is now obsoleted by
NGHTTP2_CLIENT_CONNECTION_PREFACE.

pending_local_max_concurrent_stream is now set in
nghttp2_session_add_settings, rather than after frame was sent.