Support the "%u" username alias in PlainUsers

[casantos]

This permits to enable PAM for the effective user of the Xvnc process by adding this to ~/.vnc/config or /etc/tigervnc/vncserver-config-defaults:

SecurityTypes=TLSPlain
PlainUsers=%u

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2233204

[CendioOssman]

This looks nice and clean! Thanks!

I wonder if there is a more descriptive marker we can use, though. I understand why systemd uses a single character, as it behaves more like a format string there. But in our case, it's a complete entry.

Did you stumble upon any more examples of configuration that has a marker like this? And wasn't using $USER. :)

%u probably works if we can't find anything better. But this is something we can't easily change later, so I think it is worth looking a bit extra at what options we have here.

[CendioOssman]

Dovecot also uses %u for the current user:

https://doc.dovecot.org/configuration_manual/config_file/config_variables/#variables-mail-user

It also has longer names in the notation %{user}. That syntax is also used by Apache's mod_rewrite:

https://httpd.apache.org/docs/2.4/mod/mod_rewrite.html

.desktop files unfortunately use %u for URLs:

https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html

Not that I think that is a blocker.

[casantos]

My first draft recognized$USER and $LOGNAME but I switched to %u to avoid confusing the parameter value with an environment variable.

I think %(user} is unnecessarily verbose. BASH uses \u for the prompt definitions but I don't consider \ better than %.

[CendioOssman]

We don't seem to find any better alternatives, so let's proceed with %u.