A helper function for dealing with ALPN in server side.  The *in*
contains peer's protocol list in preferable order.  The format of
*in* is length-prefixed and not null-terminated.  For example,
``h2`` and ``http/1.1`` stored in *in* like this::

    in[0] = 2
    in[1..2] = "h2"
    in[3] = 8
    in[4..11] = "http/1.1"
    inlen = 12

The selection algorithm is as follows:

1. If peer's list contains HTTP/2 protocol the library supports,
   it is selected and returns 1. The following step is not taken.

2. If peer's list contains ``http/1.1``, this function selects
   ``http/1.1`` and returns 0.  The following step is not taken.

3. This function selects nothing and returns -1 (So called
   non-overlap case).  In this case, *out* and *outlen* are left
   untouched.

Selecting ``h2`` means that ``h2`` is written into *\*out* and its
length (which is 2) is assigned to *\*outlen*.

For ALPN, refer to https://tools.ietf.org/html/rfc7301

To use this method you should do something like::

    static int alpn_select_proto_cb(SSL* ssl,
                                    const unsigned char **out,
                                    unsigned char *outlen,
                                    const unsigned char *in,
                                    unsigned int inlen,
                                    void *arg)
    {
        int rv;
        rv = nghttp2_select_alpn(out, outlen, in, inlen);
        if (rv == -1) {
            return SSL_TLSEXT_ERR_NOACK;
        }
        if (rv == 1) {
            ((MyType*)arg)->http2_selected = 1;
        }
        return SSL_TLSEXT_ERR_OK;
    }
    ...
    SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, my_obj);