core: Implement setname

Implement the IRCv3 "setname" capability, allowing Quassel to receive
updates to other nicknames' realnames, similar to "away-notify".

Add the "/setname" command to allow setting one's own realname without
reconnecting.  This requires the server to support "setname".

[Original commit by justJanne, tweaked by digitalcircuit - setname is
now ratified and official, hooray!]

See https://ircv3.net/specs/extensions/setname