Creates an RPC server handle using the UDP transport, to
    which it returns a pointer. Upon completion, xprt->xp_sock is
    the transport's socket descriptor, and xprt->xp_port is the
    transport's port number. The service is automatically registered
    as a transporter (thereby including its socket in svc_fds such
    that its socket descriptor is included in all RPC select system
    calls).