Add varlink interface for sysinstall (#42261)

Allow graphical installers to use system sysinstall. This will be used
by gnome-setup to install GNOME OS and ideally installers of other
distributions talk to the same varlink interface.

src/sysinstall/io.systemd.sysinstall.policy

@@ -0,0 +1,40 @@
+<?xml version="1.0" encoding="UTF-8"?> <!--*-nxml-*-->
+<!DOCTYPE policyconfig PUBLIC "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
+        "https://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd">
+
+<!--
+  SPDX-License-Identifier: LGPL-2.1-or-later
+
+  This file is part of systemd.
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+-->
+
+<policyconfig>
+
+        <vendor>The systemd Project</vendor>
+        <vendor_url>https://systemd.io</vendor_url>
+
+        <action id="io.systemd.sysinstall.Run">
+                <description gettext-domain="systemd">Install OS to disk</description>
+                <message gettext-domain="systemd">Authentication is required to install an OS to disk.</message>
+                <defaults>
+                        <allow_any>auth_admin</allow_any>
+                        <allow_inactive>auth_admin</allow_inactive>
+                        <allow_active>auth_admin_keep</allow_active>
+                </defaults>
+        </action>
+
+        <action id="io.systemd.sysinstall.ListCandidateDevices">
+                <description gettext-domain="systemd">List candidate devices</description>
+                <message gettext-domain="systemd">Authentication is required to list device candidates.</message>
+                <defaults>
+                        <allow_any>auth_admin</allow_any>
+                        <allow_inactive>auth_admin</allow_inactive>
+                        <allow_active>auth_admin_keep</allow_active>
+                </defaults>
+        </action>
+</policyconfig>

src/sysinstall/meson.build

@@ -8,3 +8,8 @@ executables += [
                 'sources' : files('sysinstall.c'),
         },
 ]
+
+if conf.get('ENABLE_SYSINSTALL') == 1
+        install_data('io.systemd.sysinstall.policy',
+                     install_dir : polkitpolicydir)
+endif