GNOME prototype: detect GNOME/GDM, GDM autologin and session switching

patches/steamify-gdm-relogin.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# Log the autologin user in again, into the session steam-set-session picked
+# (Steamify prototype, GDM). Run as root through sudo, from inside the session
+# that is being left. GDM logs in by itself only when the daemon starts, and
+# a new session refuses to start while the old user manager still runs
+# ("A graphical session is already running!"): so the user is ended first,
+# then GDM restarted. That work runs in its own transient unit, because
+# ending the user kills this process too.
+if [[ "$1" != --work ]]; then
+    [[ -n "${SUDO_USER:-}" ]] || exit 1
+    exec systemd-run --no-block --collect --unit=steamify-gdm-relogin "$0" --work "$SUDO_USER"
+fi
+USER_NAME="$2"
+# No waiting for the old user manager to go: systemd starts the new login
+# after it, and waiting only keeps GDM's login screen up in the meantime.
+loginctl terminate-user "$USER_NAME"
+systemctl restart gdm.service