selinux: Permit using systemd-userdbd

[debarshiray]

The systemd-userdbd service was added in systemd 245, which was
released in March 2020 and is available in RHEL 9. Therefore, it's
safe to assume that the systemd_userdbd_stream_connect() SELinux
interface is also available on all relevant operating systems, unless
there's reason to believe otherwise.

https://bugzilla.redhat.com/show_bug.cgi?id=2071217

[debarshiray]

I don't know much about systemd-userdbd beyond reading what's there in the manuals, and I don't know exactly how or why the flatpak-system-helper might be using it, or what happens when it can't do so.

I suppose flatpak-system-helper is using systemd-userdbd for the same reason that it tries to read /etc/passwd, and this is happening through nss-systemd, and it falls back to reading /etc/passwd when that fails. Is that right?

[debarshiray]

Ping @zpytela

[debarshiray]

Thanks, @alexlarsson