keyfile: write the gateway explicitly

The keyfile format allows to specify the gateway in two ways: with a
separate "gateway" key, or by appending the gateway address to one of
the address$N lines:

  [ipv4]
  address1=192.0.2.1/24
  gateway=192.0.2.254

  [ipv4]
  address1=192.0.2.1/24,192.0.2.254

The former syntax is self-documenting and easier to understand for
users, but NetworkManager defaults to the latter when writing
connection files, for historical reasons. Change that and use the
explicit form.

Note that if a users has scripts manually parsing keyfiles, they could
stop working and so this can be considered an API breakage. OTOH,
those scripts are buggy if they don't support both forms, and they can
already break with perfectly valid user-generated keyfiles.

I think it's acceptable to change the default way to persist keyfiles;
the only precaution would be that this patch should not be applied
during a stable release cycle of a distro.

src/core/settings/plugins/keyfile/tests/keyfiles/Test_Write_Bridge_Main

@@ -9,7 +9,8 @@ interface-name=br0
 [bridge]
 
 [ipv4]
-address1=1.2.3.4/24,1.1.1.1
+address1=1.2.3.4/24
+gateway=1.1.1.1
 method=manual
 
 [ipv6]

src/core/settings/plugins/keyfile/tests/keyfiles/Test_Write_Wired

@@ -10,9 +10,10 @@ mac-address=99:88:77:66:55:44
 mtu=900
 
 [ipv4]
-address1=192.168.0.5/24,192.168.0.1
+address1=192.168.0.5/24
 address2=1.2.3.4/8
 dns=4.2.2.1;4.2.2.2;
+gateway=192.168.0.1
 method=manual
 route1=10.10.10.2/24,10.10.10.1,3
 route2=1.1.1.1/8,1.2.1.1,1

src/core/settings/plugins/keyfile/tests/keyfiles/Test_Write_Wired_IP6

@@ -11,9 +11,10 @@ method=disabled
 
 [ipv6]
 addr-gen-mode=default
-address1=abcd::beef/64,dcba::beef
+address1=abcd::beef/64
 dns=1::cafe;
 dns-search=wallaceandgromit.com;
+gateway=dcba::beef
 method=manual
 
 [proxy]

src/libnm-core-impl/nm-keyfile.c

@@ -2263,11 +2263,7 @@ ip6_addr_gen_mode_writer(KeyfileWriterInfo *info,
 }
 
 static void
-write_ip_values(GKeyFile   *file,
+write_ip_values(GKeyFile *file, const char *setting_name, GPtrArray *array, gboolean is_route)
-                const char *setting_name,
-                GPtrArray  *array,
-                const char *gateway,
-                gboolean    is_route)
 {
     if (array->len > 0) {
         nm_auto_str_buf NMStrBuf output = NM_STR_BUF_INIT(2 * INET_ADDRSTRLEN + 10, FALSE);
@@ -2300,7 +2296,7 @@ write_ip_values(GKeyFile   *file,
 
                 addr = nm_ip_address_get_address(address);
                 plen = nm_ip_address_get_prefix(address);
-                gw   = (i == 0) ? gateway : NULL;
+                gw   = NULL;
             }
 
             nm_str_buf_set_size(&output, 0, FALSE, FALSE);
@@ -2351,11 +2347,10 @@ addr_writer(KeyfileWriterInfo *info, NMSetting *setting, const char *key, const
 {
     GPtrArray  *array;
     const char *setting_name = nm_setting_get_name(setting);
-    const char *gateway      = nm_setting_ip_config_get_gateway(NM_SETTING_IP_CONFIG(setting));
 
     array = (GPtrArray *) g_value_get_boxed(value);
     if (array && array->len)
-        write_ip_values(info->keyfile, setting_name, array, gateway, FALSE);
+        write_ip_values(info->keyfile, setting_name, array, FALSE);
 }
 
 static void
@@ -2366,7 +2361,7 @@ route_writer(KeyfileWriterInfo *info, NMSetting *setting, const char *key, const
 
     array = (GPtrArray *) g_value_get_boxed(value);
     if (array && array->len)
-        write_ip_values(info->keyfile, setting_name, array, NULL, TRUE);
+        write_ip_values(info->keyfile, setting_name, array, TRUE);
 }
 
 static void
@@ -3060,7 +3055,6 @@ static const ParseInfoSetting *const parse_infos[_NM_META_SETTING_TYPE_NUM] = {
                                 .parser              = ip_dns_parser,
                                 .writer              = dns_writer, ),
             PARSE_INFO_PROPERTY(NM_SETTING_IP_CONFIG_DNS_OPTIONS, .always_write = TRUE, ),
-            PARSE_INFO_PROPERTY(NM_SETTING_IP_CONFIG_GATEWAY, .writer_skip = TRUE, ),
             PARSE_INFO_PROPERTY(NM_SETTING_IP_CONFIG_ROUTES,
                                 .parser_no_check_key = TRUE,
                                 .parser              = ip_address_or_route_parser,
@@ -3088,7 +3082,6 @@ static const ParseInfoSetting *const parse_infos[_NM_META_SETTING_TYPE_NUM] = {
                                 .parser              = ip_dns_parser,
                                 .writer              = dns_writer, ),
             PARSE_INFO_PROPERTY(NM_SETTING_IP_CONFIG_DNS_OPTIONS, .always_write = TRUE, ),
-            PARSE_INFO_PROPERTY(NM_SETTING_IP_CONFIG_GATEWAY, .writer_skip = TRUE, ),
             PARSE_INFO_PROPERTY(NM_SETTING_IP_CONFIG_ROUTES,
                                 .parser_no_check_key = TRUE,
                                 .parser              = ip_address_or_route_parser,