mute-console: add simple varlink service that can disable log/status spew to kernel console

For "wizard" style interactive tools it's very annoying if they are
interrupted by kernel log output or PID1's status output. let's add some
infra to disable this temporarily. I decided to implement this as an IPC
service so that we can make this robust: if the client request the
muting dies we can automatically unmute again.

This is hence a tiny varlink service, but it can also be started
directly from the cmdline.

units/meson.build

@@ -143,6 +143,12 @@ units = [
         },
         { 'file' : 'modprobe@.service' },
         { 'file' : 'multi-user.target' },
+        {
+          'file' : 'systemd-mute-console.socket',
+          'symlinks' : ['sockets.target.wants/']
+        },
+        { 'file' : 'systemd-mute-console@.service' },
+        { 'file' : 'system-systemd\\x2dmute\\x2dconsole.slice' },
         { 'file' : 'network-online.target' },
         { 'file' : 'network-pre.target' },
         { 'file' : 'network.target' },

units/system-systemd\x2dmute\x2dconsole.slice

@@ -0,0 +1,19 @@
+#  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=Console Output Muting Service Slice
+Documentation=man:systemd-mute-console(8)
+DefaultDependencies=no
+Conflicts=shutdown.target
+Before=shutdown.target
+
+[Slice]
+# Serialize requests to mute the console.
+ConcurrencySoftMax=1

units/systemd-mute-console.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=Console Output Muting Service Socket
+Documentation=man:systemd-mute-console(8)
+DefaultDependencies=no
+Before=sockets.target
+Conflicts=shutdown.target
+Before=shutdown.target
+
+[Socket]
+ListenStream=/run/systemd/io.systemd.MuteConsole
+FileDescriptorName=varlink
+SocketMode=0600
+Accept=yes

units/systemd-mute-console@.service

@@ -0,0 +1,18 @@
+#  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=Console Output Muting Service
+Documentation=man:systemd-mute-console(8)
+DefaultDependencies=no
+Conflicts=shutdown.target
+Before=shutdown.target
+
+[Service]
+ExecStart=systemd-mute-console