revoke write authority

[zooko]

As described in ​http://allmydata.org/pipermail/tahoe-dev/2009-June/001995.html , the easiest kind of revocation to implement in a distributed, robust way is also the kind of revocation that I most urgently need: revoke the write-authority embodied in a specific cap.

The way to implement this is to define a special out-of-band symbol (i.e., something unambiguously distinct from file contents) which means "this file has been petrified". That would be a way to take a mutable file and turn it into a petrified file (formerly mutable but now immutable).