Performs post-process of HTTP Upgrade request.  This function can
be called from both client and server, but the behavior is very
different in each other.

If called from client side, the *settings_payload* must be the
value sent in ``HTTP2-Settings`` header field and must be decoded
by base64url decoder.  The *settings_payloadlen* is the length of
*settings_payload*.  The *settings_payload* is unpacked and its
setting values will be submitted using `nghttp2_submit_settings()`.
This means that the client application code does not need to submit
SETTINGS by itself.  The stream with stream ID=1 is opened and the
*stream_user_data* is used for its stream_user_data.  The opened
stream becomes half-closed (local) state.

If called from server side, the *settings_payload* must be the
value received in ``HTTP2-Settings`` header field and must be
decoded by base64url decoder.  The *settings_payloadlen* is the
length of *settings_payload*.  It is treated as if the SETTINGS
frame with that payload is received.  Thus, callback functions for
the reception of SETTINGS frame will be invoked.  The stream with
stream ID=1 is opened.  The *stream_user_data* is ignored.  The
opened stream becomes half-closed (remote).

If the request method is HEAD, pass nonzero value to
*head_request*.  Otherwise, pass 0.

This function returns 0 if it succeeds, or one of the following
negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    The *settings_payload* is badly formed.
:enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
    The stream ID 1 is already used or closed; or is not available.