Add command header to sessiond->client response msg

Add command header to sessiond->client response msg

Response messages from the session daemon have this layout
for the moment:

    [message header] (fixed size)
    [payload data]   (0 to n bytes)

This patch makes them have this layout:

    [message header] (fixed size)
    [command header] (0 to n bytes)
    [payload data]   (0 to n bytes)

The command header allows a command to specify additional
informations about its payload.

The header size is set to 0 for all commands for the moment.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>