Implement IRCv3.2 capability negotiation

Implement IRCv3.2 capability negotiation

Always send CAP LS, similar to ZNC and HexChat.  IrcEventCap checks
returned capabilities, queuing up desired ones until all replies
finish, then requests each capability one-by-one.  This prevents an
error with one capability from blocking all of them.

See http://ircv3.net/specs/core/capability-negotiation-3.1.html
And http://ircv3.net/specs/core/capability-negotiation-3.2.html

Modify SASL authentication to only happen if present in capability
list.  If it's not supported, log an error in the status buffer and
continue connecting anyways.

See http://ircv3.net/specs/extensions/sasl-3.1.html
And http://ircv3.net/specs/extensions/sasl-3.2.html