rename oragono.io/bnc and `bouncer` config section

[slingamn]

"Bouncer" is confusing nomenclature for a feature that only provides sharing of nicks. It also conflicts with kiwibnc's planned "bouncer" cap.

However, "nicksharing" is not a good name. Maybe "multiclient"?

It should be easy to keep supporting old config files that use the bouncer name.

[slingamn]

This is also exposed as the name of an account setting.

[slingamn]

Given that we now recommend server operators make multiclient an opt-out feature, and that there is no way to opt out via the cap system as yet, it's unclear that multiclient even needs to be a cap at all.

[slingamn]

Closed in #792