Add better named key to allow account names to differ from nicks

[DanielOaks]

Also bumps the cap name since this differs from the current syntax.

[kaniini]

I'm willing to work on getting this done in both Atheme and Solanum in present state.

[progval]

For context, @kaniini pointed out this is a requirement for Ophion and for Atheme's UserServ mode. Some services (ratbox services, QuakeNet, ...) would also need something like this if they were to ever implement this spec.

Additional complexity is minimal, as servers not interested in it can just reject it, and clients can use *. The only downside is that people using /register as a raw command need to add * to the arg list