Currently the realname is set to an advertisement if it isn't explicitly
set by the user.
Some clients started to show the realname as a display name in their
UI, which makes this tedious as you'll end up with gazillion "The Lounge
User" entries.
To avoid this, set the realname to the nick on first connect, so that
it is useful.
Note that this isn't done on nick changes, but only on the initial
connect step.
Fixes: https://github.com/thelounge/thelounge/issues/4527
