BUG/MEDIUM: hlua/cli: Fix lua CLI commands to work with applet's buffers

BUG/MEDIUM: hlua/cli: Fix lua CLI commands to work with applet's buffers

In 3.0, the CLI applet was rewritten to use its own buffers. However, the
lua part, used to register CLI commands at runtime, was not updated
accordingly. It means the lua CLI commands still try to write in the channel
buffers. This is of course totally unexepected and not supported. Because of
this bug, the applet hangs intead of returning the command result.

The registration of lua CLI commands relies on the lua TCP applets. So the
send and receive functions were fixed to use the applet's buffer when it is
required and still use the channel buffers otherwies. This way, other lua
TCP applets can still run on the legacy mode, without the applet's buffers.

This patch must be backported to 3.0.

(cherry picked from commit e5e36ce09722e63ca4542b0b2ff1a1eb905f8208)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>