report: move facts generator out of PID1 into a separate varlink service

The collection of facts is entirely unprivileged and has very little to
do with PID1. PID1 is privileged and single-threaded and a point of
contention, so we shouldn't put things in PID1 that don't need to be
there. A separate service can be enabled/disabled/started/stopped at
will, is easy to sandbox, etc. If it turns out to be necessary to
collect some facts through PID1 in the future, we can always add a
smaller facts endpoint to PID1 again.

units/systemd-report-basic.socket

@@ -0,0 +1,22 @@
+# 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.
+[Unit]
+Description=Report System Basic Facts Socket
+DefaultDependencies=no
+Before=sockets.target
+
+[Socket]
+ListenStream=/run/systemd/report/io.systemd.Basic
+FileDescriptorName=varlink
+SocketMode=0666
+Accept=yes
+RemoveOnStop=yes
+
+[Install]
+WantedBy=sockets.target