networkd: use socket activation when starting networkd

networkd: use socket activation when starting networkd

Add After=systemd-networkd.socket to avoid a race condition and networkd
falling back to the non-socket activation code.

Also add Wants=systemd-networkd.socket, so the socket is started when
networkd is started via `systemctl start systemd-networkd.service`.
A Requires is not strictly necessary, as networkd still ships the
non-socket activation code. Should this code be removed one day, the
Wants should be bumped to Requires accordingly.

See also 5544ee85163733eaa50f598fcf3bd9421d4a42f9.

Fixes: #16809