ifcfg-rh: fix persisting all-default NMSettingEthtool settings

We somehow need to encode an NMSettingEthtool instance that has all
options unset. Previously, that would result in no "$ETHTOOL_OPTS"
variable and thus the reader would loose a previously existing setting.

Hack it by writing a bogus

  ETHTOOL_OPTS="-A $IFACE"

line.

src/core/settings/plugins/ifcfg-rh/nms-ifcfg-rh-writer.c

@@ -1376,6 +1376,13 @@ write_ethtool_setting(NMConnection *connection, shvarFile *ifcfg, GError **error
             g_string_append(str, nms_ifcfg_rh_utils_get_ethtool_name(ethtool_id));
             g_string_append(str, b ? " on" : " off");
         }
+
+        if (!str) {
+            /* Write an empty dummy "-A" option without arguments. This is to
+             * ensure that the reader will create an (all default) NMSettingEthtool.
+             * Also, it seems that `ethtool -A "$IFACE"` is silently accepted. */
+            _ethtool_gstring_prepare(&str, &is_first, 'A', iface);
+        }
     }
 
     if (str) {

src/core/settings/plugins/ifcfg-rh/tests/network-scripts/ifcfg-test_roundtrip_ethtool-2.cexpected

@@ -1,6 +1,7 @@
 TYPE=Ethernet
 PROXY_METHOD=none
 BROWSER_ONLY=no
+ETHTOOL_OPTS="-A net0"
 BOOTPROTO=dhcp
 DEFROUTE=yes
 IPV4_FAILURE_FATAL=no

src/core/settings/plugins/ifcfg-rh/tests/test-ifcfg-rh.c

@@ -3969,7 +3969,8 @@ test_roundtrip_ethtool(void)
                                                  NULL,
                                                  NM_SETTING_WIRED_SETTING_NAME,
                                                  NULL);
-    /* TODO: add empty NMSettingEthtool, which currently would break the test. */
+    s_ethtool  = nm_setting_ethtool_new();
+    nm_connection_add_setting(connection, s_ethtool);
     _writer_new_connec_exp(connection,
                            TEST_SCRATCH_DIR,
                            TEST_IFCFG_DIR "/ifcfg-test_roundtrip_ethtool-2.cexpected",