MINOR: stconn: Start to introduce mux-to-mux fast-forwarding notion

MINOR: stconn: Start to introduce mux-to-mux fast-forwarding notion

Instead of talking about kernel splicing at stconn/sedesc level, we now try
to talk about mux-to-mux fast-forwarding. To do so, 2 functions were added
to know if there are fast-forwarded data and to retrieve this amount of
data. Of course, for now, there is only data in a pipe.

In addition, some flags were renamed to reflect this notion. Note the
channel's documentation was not updated yet.