mesh: Add unit test IO

mesh: Add unit test IO

This adds a new type of mesh IO that is used for non-interactive testing.
The new io option can be specified on command line as:
--io unit:<socket_name>

When the bluetooth-meshd daemon starts with the "unit" IO type,
the daemon opens a socket (fd to open is provided after "unit:"
in <socket_name>). The communication with the daemon is done either
through the loop-back using mesh DBus-based APIs or the specified
named socket.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>