network: DHCP server Add support to transmit SIP server

1. DHCP server trasmit
2. Client parses and saves in leases
Implements http://www.rfc-editor.org/rfc/rfc3361.txt

```
Frame 134: 348 bytes on wire (2784 bits), 348 bytes captured (2784 bits) on interface 0
Ethernet II, Src: 42:65:85:d6:4e:32 (42:65:85:d6:4e:32), Dst: 1e:04:f8:b8:2f:d4 (1e:04:f8:b8:2f:d4)
Internet Protocol Version 4, Src: 192.168.5.1, Dst: 192.168.5.11
User Datagram Protocol, Src Port: 67, Dst Port: 68
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x7cc87cb4
    Seconds elapsed: 0
    Bootp flags: 0x0000 (Unicast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 192.168.5.11
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: 1e:04:f8:b8:2f:d4 (1e:04:f8:b8:2f:d4)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
        Length: 1
        DHCP: ACK (5)
    Option: (51) IP Address Lease Time
        Length: 4
        IP Address Lease Time: (3600s) 1 hour
    Option: (1) Subnet Mask (255.255.255.0)
        Length: 4
        Subnet Mask: 255.255.255.0
    Option: (3) Router
        Length: 4
        Router: 192.168.5.1
    Option: (6) Domain Name Server
        Length: 4
        Domain Name Server: 192.168.5.1
    Option: (42) Network Time Protocol Servers
        Length: 4
        Network Time Protocol Server: 192.168.1.1
    Option: (120) SIP Servers <=====here
        Length: 9
        SIP Server Encoding: IPv4 Address (1)
        SIP Server Address: 192.168.1.1
        SIP Server Address: 192.168.5.2
    Option: (101) TCode
        Length: 13
        TZ TCode: Europe/Berlin
    Option: (54) DHCP Server Identifier (192.168.5.1)
        Length: 4
        DHCP Server Identifier: 192.168.5.1
    Option: (255) End
        Option End: 255
```

```
cat /run/systemd/netif/state                                                                                                   ✔  ⚡  3148  16:40:51
OPER_STATE=routable
CARRIER_STATE=carrier
ADDRESS_STATE=routable
DNS=192.168.94.2 192.168.5.1
NTP=192.168.5.1
SIP=192.168.1.1 192.168.5.2

```

aa

1 files changed, 21 insertions, 0 deletions

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index 0fbc4dfe26..585041095d 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -1349,6 +1349,14 @@
           </listitem>
         </varlistentry>
         <varlistentry>
+          <term><varname>UseSIP=</varname></term>
+          <listitem>
+            <para>When true (the default), the SIP servers received
+            from the DHCP server will be saved at the state files and can be
+            read via <function>sd_network_link_get_sip_servers()</function> function.</para>
+          </listitem>
+        </varlistentry>
+        <varlistentry>
           <term><varname>UseMTU=</varname></term>
           <listitem>
             <para>When true, the interface maximum transmission unit
@@ -1777,6 +1785,19 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>EmitSIP=</varname></term>
+        <term><varname>SIP=</varname></term>
+
+        <listitem><para>Similar to the <varname>EmitDNS=</varname> and
+        <varname>DNS=</varname> settings described above, these
+        settings configure whether and what SIP server information
+        shall be emitted as part of the DHCP lease. The same syntax,
+        propagation semantics and defaults apply as for
+        <varname>EmitDNS=</varname> and
+        <varname>DNS=</varname>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>EmitRouter=</varname></term>
 
         <listitem><para>Similar to the <varname>EmitDNS=</varname>