userdbctl: enable ssh-authorized-keys logic by default

sshd now supports config file drop-ins, hence let's install one to hook
up "userdb ssh-authorized-keys", so that things just work.

We put the drop-in relatively early, so that other drop-ins generally
will override this.

Ideally sshd would support such drop-ins in /usr/ rather than /etc/, but
let's take what we can get. It's not that sshd's upstream was
particularly open to weird ideas from Linux people.

tmpfiles.d/20-systemd-userdb.conf.in

@@ -0,0 +1,10 @@
+#  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.
+
+# See tmpfiles.d(5) for details
+
+L {{SSHDCONFDIR}}/20-systemd-userdb.conf - - - - {{LIBEXECDIR}}/sshd_config.d/20-systemd-userdb.conf

tmpfiles.d/meson.build

@@ -34,6 +34,7 @@ in_files = [['etc.conf',                      ''],
             ['static-nodes-permissions.conf', ''],
             ['systemd.conf',                  ''],
             ['var.conf',                      ''],
+            ['20-systemd-userdb.conf',        'ENABLE_USERDB'],
            ]
 
 foreach pair : in_files