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, 36 insertions, 0 deletions

diff --git a/src/libsystemd-network/sd-dhcp-server.c b/src/libsystemd-network/sd-dhcp-server.c
index 7cb44d1fdf..13f104f9ef 100644
--- a/src/libsystemd-network/sd-dhcp-server.c
+++ b/src/libsystemd-network/sd-dhcp-server.c
@@ -139,6 +139,7 @@ static sd_dhcp_server *dhcp_server_free(sd_dhcp_server *server) {
         free(server->timezone);
         free(server->dns);
         free(server->ntp);
+        free(server->sip);
 
         hashmap_free(server->leases_by_client_id);
 
@@ -498,6 +499,15 @@ static int server_send_ack(sd_dhcp_server *server, DHCPRequest *req,
                         return r;
         }
 
+        if (server->n_sip > 0) {
+                r = dhcp_option_append(
+                                &packet->dhcp, req->max_optlen, &offset, 0,
+                                SD_DHCP_OPTION_SIP_SERVER,
+                                sizeof(struct in_addr) * server->n_sip, server->sip);
+                if (r < 0)
+                        return r;
+        }
+
         if (server->timezone) {
                 r = dhcp_option_append(
                                 &packet->dhcp, req->max_optlen, &offset, 0,
@@ -1124,6 +1134,32 @@ int sd_dhcp_server_set_ntp(sd_dhcp_server *server, const struct in_addr ntp[], u
         return 1;
 }
 
+int sd_dhcp_server_set_sip(sd_dhcp_server *server, const struct in_addr sip[], unsigned n) {
+        assert_return(server, -EINVAL);
+        assert_return(sip || n <= 0, -EINVAL);
+
+        if (server->n_sip == n &&
+            memcmp(server->sip, sip, sizeof(struct in_addr) * n) == 0)
+                return 0;
+
+        if (n <= 0) {
+                server->sip = mfree(server->sip);
+                server->n_sip = 0;
+        } else {
+                struct in_addr *c;
+
+                c = newdup(struct in_addr, sip, n);
+                if (!c)
+                        return -ENOMEM;
+
+                free(server->sip);
+                server->sip = c;
+                server->n_sip = n;
+        }
+
+        return 1;
+}
+
 int sd_dhcp_server_set_emit_router(sd_dhcp_server *server, int enabled) {
         assert_return(server, -EINVAL);