Register DNS server in systemd-resolved

When configuring DNS, do so in resolved too, not only plain
/etc/resolv.conf. The former should load the latter, but it saves one
indirection (resolved is used via NSS directly, at least on Fedora, even
if /etc/resolv.conf points somewhere else).

Related to QubesOS/qubes-issues#7429

network/setup-ip

@@ -60,6 +60,11 @@ configure_network() {
             echo "nameserver $secondary_dns" >> /etc/resolv.conf
         fi
     fi
+    if [ -x /usr/bin/resolvectl ] && \
+            systemctl is-enabled -q systemd-resolved.service && \
+            ! qsvc disable-dns-server ; then
+        resolvectl dns "$INTERFACE" "$primary_dns" "$secondary_dns"
+    fi
 }
 
 configure_network_nm() {