support proper user account tracking via IRCv3 extensions

[mattzque]

Implement IRCv3 extensions that enable accurate user account tracking. This is useful to properly authenticate users:

• https://ircv3.net/specs/extensions/account-notify-3.1
• https://ircv3.net/specs/extensions/extended-join-3.1
• https://ircv3.net/specs/extensions/multi-prefix-3.1