add SPLICE_F_MORE to lttd splice

    SPLICE_F_MORE      More  data  will  be  coming in a subsequent splice.
                       This is a helpful hint when the fd_out refers  to  a
                       socket  (see  also  the  description  of MSG_MORE in
                       send(2), and the description of TCP_CORK in tcp(7))

Will be a useful guideline for network sending daemons.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>