Initializes *\*session_ptr* for client use.  The all members of
*callbacks* are copied to *\*session_ptr*.  Therefore *\*session_ptr*
does not store *callbacks*.  The *user_data* is an arbitrary user
supplied data, which will be passed to the callback functions.

The :type:`nghttp2_send_callback2` must be specified.  If the
application code uses `nghttp2_session_recv()`, the
:type:`nghttp2_recv_callback` must be specified.  The other members
of *callbacks* can be ``NULL``.

If this function fails, *\*session_ptr* is left untouched.

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

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.