units: use Requires in systemd-networkd-wait-online.service (#6065)

units: use Requires in systemd-networkd-wait-online.service (#6065)

In the initial design, foobar-wait-online.service would have
Requisite=foobar.service, so that foobar-wait-online.service could be enabled
unconditionally, irrespective of whether foobar.service itself is enabled.
Unfortunately this doesn't work too well:

1. the message about foobar-wait-online.service being skipped because of a
   "missing dependency" *looks* like an is problem. This is mostly cosmetic,
   but it also quite confusing. We generally don't want any messages of this
   type during default boot.

2. it is impossible to start and wait for the network in an
   implementation-agnostic way: systemctl start network-online.target, or
   Wants/After=network-online.target in a unit don't work because pulling in
   network-online.target pulls in foobar-wait-online.service, but it in turn
   does not pull in foobar.service. During startup, foobar.service is pulled in
   by multi-user.target, but not in a smaller transaction which does not
   include multi-user.target.

This change means that *-wait-online.service should be installed through
presets, so that it can be enabled/disabled at will by the administrator.
Our own systemd-networkd-wait-online.service does this already, and
similar change has been requested for NetworkManager-wait-online.service
(https://bugzilla.redhat.com/show_bug.cgi?id=1455704).

This change should by mostly backwards-compatible, unless somebody has some
wait-online.service enabled, without having the corresponding network
implementation enabled, and they are relying on it not being started.  I think
that's relatively unlikely because of issue 1. above, and I'm not aware of this
being the default in any distro. And being able to start the network in an
implementation-agnostic way is pretty important, see
https://bugzilla.redhat.com/show_bug.cgi?id=1452866.