Submits ALTSVC frame.

ALTSVC frame is a non-critical extension to HTTP/2, and defined in
`RFC 7383 <https://tools.ietf.org/html/rfc7838#section-4>`_.

The *flags* is currently ignored and should be
:enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.

The *origin* points to the origin this alternative service is
associated with.  The *origin_len* is the length of the origin.  If
*stream_id* is 0, the origin must be specified.  If *stream_id* is
not zero, the origin must be empty (in other words, *origin_len*
must be 0).

The ALTSVC frame is only usable from server side.  If this function
is invoked with client side session, this function returns
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.

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_STATE`
    The function is called from client side session
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
    The sum of *origin_len* and *field_value_len* is larger than
    16382; or *origin_len* is 0 while *stream_id* is 0; or
    *origin_len* is not 0 while *stream_id* is not 0.