Anonymous and/or client gateway closures

[jcnelson]

Allow anonymous or client gateways to have closures.

• add an MS handler for getting the "default" closure (i.e. by volume)
• add a way for syntool to set one for a volume

[jcnelson]

Will be fixed by making drivers content-addressable, and separately downloadable. The code to verify will check to see if the gateway is anonymous, and if so, check for the volume owner's signature.

[jcnelson]

Drivers are content-addressable in my own tree, and driver signatures are checked against the volume owner for anonymous gateways.. Just need to evaluate.