Fix a double-free in the Gtk event log.

It could occur some time after a line was selected in the event log
window.
Merge Gtk event log fix from 'pre-0.64'.
Document IPv6 literal syntax for tunnels.
Merge another docs fix from 'pre-0.64'.
Disable manual host key config in mid-session.

Changing it can't have any useful effect, since we have strictly
enforced that the host key used for rekeys is the same as the first key
exchange since b8e668c.
Disable some mid-session configs for downstreams.

Compression, encryption, and key exchange settings are all meaningless
to reconfigure in connection-sharing downstreams.
Merge reconfig fixes from branch 'pre-0.64'.