Allow channel mode override in snapshot sessions

Allow the lttng cmd line and liblttng-ctl users to override the channel
mode in snapshot sessions.

Note that liblttng-ctl users expecting that an "overwrite" mode
explicitly set at the value "0" (discard) for a snapshot session will
now create a channel in discard mode.

The DEFAULT_CHANNEL_OVERWRITE used by liblttng-ctl
lttng_channel_set_default_attr() is changed to "-1".

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>