Replace int *eof with uint32_t *data_flags so that we can easily
extend functionality if we have to (but we don't do if possible).

To make adding new option easier, we decided to make the details of
option struct private and hide it from public API.  We provide
functions to set individual option value.

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.