Implements a new permission level PERMS_PENDING

With this a user can allow some action to any user which connects
to them, even before they've connected back.

Ref.
https://mobiliza.org.br/display/478d9e71eaf55748dc646d3990651d6d34cfb7db5c38360538ec730ca3ccf908@zothub.com

Also some code cleanup and an alternative logic for handling
notifications of permission changes in zot.php.

This assumes that private posts are still restricted to people in
your addressbook. Regardless of your global permissions, a
pending channel won't get private posts, even if the post
only has a deny clause not matching the pending channel.