MEDIUM: mux_h1/mux_pt: Use XPRT_CAN_SPLICE to decide if we should splice

MEDIUM: mux_h1/mux_pt: Use XPRT_CAN_SPLICE to decide if we should splice

In both mux_h1 and mux_pt, use the new XPRT_CAN_SPLICE capability to
decide if we should attempt to use splicing or not.
If we receive XPRT_CONN_CAN_MAYBE_SPLICE, add a new flag on the
connection, CO_FL_WANT_SPLICING, to let the xprt know that we'd love to
be able to do splicing, so that it may get ready for that.
This should have no effect right now, and is required work for adding
kTLS support.