logind: add io.systemd.Shutdown varlink interface (#41229)

The shutdown interface is currently only exposed via dbus. This PR
adds a comparable varlink implementation. It is inspired by the existing
dbus methods and implements PowerOff, Reboot, Halt, Kexec, SoftReboot
as varlink methods on a new io.systemd.Shutdown interface.

It is (intentional) simpler than the dbus for now, i.e. no Can* yet,
mostly
because I want to get feedback first (happy to do that in a followup).

The only real difficulty here is what to do about
verify_shutdown_creds()
as this is needed by both dbus and varlink and its dbus only. I went for
an ugly but (hopefully) pragmatic choice (see the commit message for
details). But I can totally understand if a refactor instead is
preferred.