add read-only mode for gateways

[zooko]

I want to host my blog and other publicly-readable documents on a grid like the Public Test Grid. The operators of the Public Test Grid gateway recently shut it down:

​http://tahoe-lafs.org/pipermail/tahoe-dev/2011-July/006572.html

A potentially good way to run the Public Test Grid, and still allow people to experiment with it, and allow me to host my blog on it, while deterring people from using it as a publishing platform for their controversial files, would be to put the public gateway into read-only mode.

I propose to add a configuration option to ​the "client" (a.k.a. "gateway") section of tahoe.cfg to make a gateway read-only.

We had talked about making it so the gateway would offer read-only service on one port and read-write service on a different port, but after more reflection I would rather not do that for now. It would be easy for users to misunderstand and think that Tahoe-LAFS was somehow going to prevent unauthorized users from using the more privileged port, when in fact the users would have to set up firewall rules and/or HTTP-level proxies themselves to prevent unauthorized users from connecting to the more privileged port. Also, I have never yet wanted a single gateway process to serve both kinds of access, so this may be a case of YAGNI. In any case, it will definitely be simpler to implement a gateway-wide read-only policy.