Recommend good default value for realname/username

[emersion]

Ref #114 (comment)

In a client, it's useful to know whether the user has set an explicit realname, or whether it's unset. If it's unset, it doesn't provide any useful information, so might as well hide it in the UI.

Some clients set the realname to the nickname if unset. This makes it easy to detect when the realname is not set to anything useful.

Some clients fill the realname with completely unrelated info like the client name. This is useless to display, a privacy issue and hard to detect.