This resolves a number of issues that were keeping communications with
foreign ships from working.
- there was no link-update mark
- there was no +on-arvo handling in link-listen-hook
- link-listen-hook was subscribing to the non-existent "link-hook"
- link-proxy-hook permission check was looking at a wrong path
Also makes link-listen-hook aware that subscriptions might get rejected
because of permissions, so that it ignores failed subscriptions instead
of being loud about it.
Stores URLs and their titles for the local ship. Can listen to
"submissions" on foreign ships.
Has a primitive perspective on groups, treating them as
always-interesting. Auto-subscribes to all ships in all groups.
Foreign communications untested.
