units: make systemd-userdbd.{socket,service} installable

It's lightweight and generally useful, so it should be enabled by default. But
users might want to disable it for whatever reason, and things should be fine
without it, so let's make it installable so it can be disabled if wanted.

Fixes #15175.

presets/90-systemd.preset

@@ -21,6 +21,7 @@ enable systemd-networkd.service
 enable systemd-resolved.service
 enable systemd-repart.service
 enable systemd-homed.service
+enable systemd-userdbd.socket
 
 disable console-getty.service
 disable debug-shell.service

units/meson.build

@@ -139,8 +139,7 @@ units = [
          'sysinit.target.wants/'],
         ['systemd-udevd-kernel.socket',         '',
          'sockets.target.wants/'],
-        ['systemd-userdbd.socket',              'ENABLE_USERDB',
-         'sockets.target.wants/'],
+        ['systemd-userdbd.socket',              'ENABLE_USERDB'],
         ['time-set.target',                     ''],
         ['time-sync.target',                    ''],
         ['timers.target',                       ''],

units/systemd-userdbd.service.in

@@ -39,3 +39,6 @@ SystemCallErrorNumber=EPERM
 SystemCallFilter=@system-service
 Type=notify
 @SERVICE_WATCHDOG@
+
+[Install]
+Also=systemd-userdbd.socket

units/systemd-userdbd.socket

@@ -17,3 +17,6 @@ Before=sockets.target
 ListenStream=/run/systemd/userdb/io.systemd.Multiplexer
 Symlinks=/run/systemd/userdb/io.systemd.NameServiceSwitch
 SocketMode=0666
+
+[Install]
+WantedBy=sockets.target