Fix account-tag in capability negotiation
Make account-notify independent from extended-join
Prefer IPv6 connections if available

This should work with IPv4-only too, but I haven't extensively tested
that.
Revert "Prefer IPv6 connections if available"

This reverts commit 0806960d57e42361c8ab4f8488bd9f4dd760d6ea.
Update README.md
Revert "Update README.md"

This reverts commit bde19769bd252dbb6e08bc76ba251bb47a28fb01.