wait-online: add support for waiting for DNS configuration
Add a new flag to systemd-networkd-wait-online, --dns, to allow waiting
for DNS to be configured.
DNS is considered configured when at least one DNS server is accessible.
If a link has the property DefaultRoute=yes (either by explicit
configuration, or because there are no routing-only domains), or if the
search domain '.' is configured, wait for link-specific DNS to be
configured. Otherwise, global DNS servers may be considered.