Fix: relayd should listen for viewers on localhost only by default

Having relayd listening by default on 0.0.0.0 (all interfaces) with a
protocol without authentication is an information leak waiting to
happen.

Users should explicitely specify if they want to listen on all
interfaces, using e.g. -L tcp://0.0.0.0:5344 (see lttng-relayd(8)
manpage for details). They should only do so if they use a firewall, or
are within a secured network.

Fixes #746

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>