networkd: add support for vxlan Remote and Local.

This patch add supports to configure IFLA_VXLAN_LOCAL
and IFLA_VXLAN_GROUP.

The "Group" is renamed to "Remote" which is a multicast address.`

```
Description=vxlan-test
Name=vxlan1
Kind=vxlan

[VXLAN]
Id=33
Local=2001:db8:2f4:4bff:fa71:1a56
Remote=FF02:0:0:0:0:0:1:9
```

output
```
ip -d link show vxlan1
16: vxlan1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1430 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether fe:b4:97:03:f8:e5 brd ff:ff:ff:ff:ff:ff promiscuity 0
    vxlan id 33 group ff02::1:9 local 2001:db8:02f4:4bff:fa71:1a56 dev enp0s3 srcport 0 0 dstport 8472 ageing 300 noudpcsum noudp6zerocsumtx noudp6zerocsumrx addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535

```

man/systemd.netdev.xml

@@ -469,12 +469,18 @@
         </listitem>
       </varlistentry>
       <varlistentry>
-        <term><varname>Group=</varname></term>
+        <term><varname>Remote=</varname></term>
         <listitem>
-          <para>An assigned multicast group IP address.</para>
+          <para>Configures destination multicast group IP address.</para>
         </listitem>
       </varlistentry>
       <varlistentry>
+        <term><varname>Local=</varname></term>
+        <listitem>
+          <para>Configures local IP address.</para>
+        </listitem>
+      </varlistentry>
+        <varlistentry>
         <term><varname>TOS=</varname></term>
         <listitem>
           <para>The Type Of Service byte value for a vxlan interface.</para>