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