pre-CAP numeric to tell a user they're going to need SASL

[jesopo]

allows clients to spring open a UI to request username and password

[jesopo]

this might be better as part of the CAP LS response

[jesopo]

this is actually unreasonably difficult because we tend to only know that someone is going to need to SASL once we've already figured out their i-line, which happens when connection registration completes