BUG/MEDIUM: mux-h1: Disable splicing for the conn-stream if read0 is received

BUG/MEDIUM: mux-h1: Disable splicing for the conn-stream if read0 is received

The CS_FL_MAY_SPLICE flag must be unset for the conn-stream if a read0 is
received while reading on the kernel pipe. It is mandatory when some data was
also received. Otherwise, this flag prevent the call to the h1 rcv_buf()
callback. Thus the read0 will never be handled by the h1 multiplexer leading to
a freeze of the session until a timeout is reached.

This patch must be backported to 2.1 and 2.0.

(cherry picked from commit a131a8fe934b40324fd8a16d237616331ea0088f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>