Serializes the SETTINGS values *iv* in the *buf*.  The size of the
*buf* is specified by *buflen*.  The number of entries in the *iv*
array is given by *niv*.  The required space in *buf* for the *niv*
entries is ``6*niv`` bytes and if the given buffer is too small, an
error is returned.  This function is used mainly for creating a
SETTINGS payload to be sent with the ``HTTP2-Settings`` header
field in an HTTP Upgrade request.  The data written in *buf* is NOT
base64url encoded and the application is responsible for encoding.

This function returns the number of bytes written in *buf*, or one
of the following negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    The *iv* contains duplicate settings ID or invalid value.

:enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
    The provided *buflen* size is too small to hold the output.