Tell Tor when we're on an IPv6-only network

This branch takes a different approach from !1294 (closed) to IPv6-only networks, as suggested by an anonymous correspondent.

If we detect that we're on an IPv6-only network, we set Tor's ClientUseIPv4 option to 0 and ClientUseIPv6 to 1. This lets us connect to the Tor network via IPv6-capable guards. If we're using bridges we switch to the meek pluggable transport, as the obfs4 bridge descriptors contain IPv4 addresses.

Closes #1794 (closed). Replaces !1294 (closed)