handle IPv6

[Brian Warner]

​Tahoe#867 is about supporting IPv6 in a tahoe grid. For that to work, we need v6 support in Foolscap. This depends upon getting v6 support in Twisted (for which you should look at ​Twisted#3014, still open as of august-2010).

Thanks to Zooko's persuasion in #60, FURLs with v6 addresses are likely to be tolerated (i.e. ignored) by current versions of Foolscap (0.2.6 or later, so everything since may-2008). In particular, any connection hint with more than one colon is ignored. So backwards compatibility shouldn't be a problem.

The FURL syntax for a v6 address would be pb://TUBID@ipv6:[fe80::1%lo0]:12345/SWISS, and the syntax for a v4+v6 FURL would be pb://TUBID@127.0.0.1:12345,ipv6:[::1]:12345/SWISS or pb://TUBID@ipv4:127.0.0.1:12345,ipv6:[::1]:12345/SWISS .

The new parsing rules will have a legacy-ipv4 rule that matches hints with a single colon, so anything else will be in the form TYPE:ADDRESS:PORT. (future schemes that don't have two-part address+port identifiers, like Tor hidden services or #151 I2P endpoints will need an extra colon, like i2p::I2PHASH.b32.i2p, to avoid being confused with legacy IPV4ADDR:PORT pairs).

Once the Twisted work is done, the foolscap-side work is a new parser in source:foolscap/referenceable.py (decode_location_hints) to spot the v6 addresses, then work in negotiate.TubConnector to stop ignoring non-ipv4 hints and call reactor.connectTCP6 or whatever the new API is. We'll also need code to parse the Tub.listenOn strports spec to handle v6, and something for setLocationAutomatically to take a v6 hint and construct a FURL from it.