When forking, ddclient redirects STDERR and STDOUT to `/dev/null`,
which prevents useful information from appearing in the systemd
journal (`journalctl`).
This improves the chances that the local DNS resolver (e.g. dnsmasq,
systemd-resolved, Unbound) is up before the service runs and avoids
DNS related failures.
Pull in network-online.target via Wants= to comply with its description
in the systemd.special(7) man page and remove the redundant
"After=network.target" since network-online.target already orders itself
after network.target.
Installing with `make install` automatically places the ddclient
executable in `/usr/bin/ddclient`. This service file has the wrong
path and that causes systemd to be unable to start ddclient.
