BUG/MINOR: mux-h1: Fix the splicing in TUNNEL mode

BUG/MINOR: mux-h1: Fix the splicing in TUNNEL mode

In the commit 17ccd1a35 ("BUG/MEDIUM: connection: add a mux flag to indicate
splice usability"), The CS_FL_MAY_SPLICE flags was added to notify the upper
layer that the mux is able to use the splicing. But this was only done for the
payload in a message, in HTTP_MSG_DATA state. But the splicing is also possible
in TUNNEL mode, in HTTP_MSG_TUNNEL state. In addition, the splicing ability is
always disabled for chunked messages.

This patch must be backported to 2.1 and 2.0.