This change allows host pattern in --backend to include '*' to
indicate wildcard match.  The wildcard match is made in suffix match
only.

This can happen when we call throwing version of
basic_stream_socket::remote_endpoint() call while client disconnected.

This option prevents the nghttp2 library from sending PING frame with
ACK flag set in the reply to incoming PING frame.  To allow the
application to send PING with ACK flag set, nghttp2_submit_ping() now
recognizes NGHTTP2_FLAG_PING in its flags parameter.