Mark greeter session as such

logind gets upset if a session is created on a VT where one already
exists, except if one of these sessions is a greeter session. Mark it as
such for robustness. It also seems more correct to do so.

greetd/src/context.rs

@@ -60,7 +60,7 @@ impl Context {
     async fn create_greeter(&self) -> Result<SessionChild, Error> {
         let mut scheduled_session = Session::new_external()?;
         scheduled_session
-            .initiate("login", "user", &self.greeter_user, false)
+            .initiate("login", "greeter", &self.greeter_user, false)
             .await?;
         match scheduled_session.get_state().await {
             Ok(SessionState::Ready) => (),