Remember last ad-hoc used

Set _last_channel so it will re-connect to the last
ad-hoc network it was connected before, during the same
sugar session.

Unset _last_channel when the user explicitly disconnects
from the ad-hoc network, so it will forget about the last
network it was connected during the same sugar session.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>