units: make enablement of s-n-wait-online.service follow systemd-networkd.service (#5635)

In 58a6dd1558 s-n-wait-online.service was added
to presets to synchronize the presets with the state after installation. But it
is harmful to have s-n-wait-online.service enabled when s-n.service is
disabled, because s-n-wait-online.service has Requsite=s-n.service and cannot
be activated. Thus remove s-n-wait-online.service from presets again, and let
it be enabled whenever s-n.service is enabled.

During installation we create enablement symlinks by hand, and since s-n.service
is enabled, s-n-w-o.service should be enabled too, so the symlink should still
be created during installation.

https://bugzilla.redhat.com/show_bug.cgi?id=1433459#c15

system-preset/90-systemd.preset

@@ -15,7 +15,6 @@ enable getty@.service
 enable systemd-timesyncd.service
 enable systemd-networkd.service
 enable systemd-resolved.service
-enable systemd-networkd-wait-online.service
 
 disable console-getty.service
 disable debug-shell.service

units/systemd-networkd.service.m4.in

@@ -42,3 +42,9 @@ ReadWritePaths=/run/systemd
 [Install]
 WantedBy=multi-user.target
 Also=systemd-networkd.socket
+
+# We want to enable systemd-networkd-wait-online.service whenever this service
+# is enabled. systemd-networkd-wait-online.service has
+# WantedBy=network-online.target, so enabling it only has an effect if
+# network-online.target itself is enabled or pulled in by some other unit.
+Also=systemd-networkd-wait-online.service