network: DHCPv6 - Add support to send user class

network: DHCPv6 - Add support to send user class

Frame 115: 171 bytes on wire (1368 bits), 171 bytes captured (1368 bits) on interface veth-peer, id 0
Ethernet II, Src: 1e:04:f8:b8:2f:d4 (1e:04:f8:b8:2f:d4), Dst: IPv6mcast_01:00:02 (33:33:00:01:00:02)
Internet Protocol Version 6, Src: fe80::1c04:f8ff:feb8:2fd4, Dst: ff02::1:2
User Datagram Protocol, Src Port: 546, Dst Port: 547
DHCPv6
    Message type: Solicit (1)
    Transaction ID: 0x673257
    Rapid Commit
        Option: Rapid Commit (14)
        Length: 0
    Identity Association for Non-temporary Address
        Option: Identity Association for Non-temporary Address (3)
        Length: 12
        Value: d0cc94090000000000000000
        IAID: d0cc9409
        T1: 0
        T2: 0
    Fully Qualified Domain Name
        Option: Fully Qualified Domain Name (39)
        Length: 6
        Value: 01045a657573
        0000 0... = Reserved: 0x00
        .... .0.. = N bit: Server should perform DNS updates
        .... ..0. = O bit: Server has not overridden client's S bit preference
        .... ...1 = S bit: Server should perform forward DNS updates
        Client FQDN: Zeus
    User Class
        Option: User Class (15)
        Length: 17
        Value: 000f68656c6c6f30313233343031323334
    Identity Association for Prefix Delegation
        Option: Identity Association for Prefix Delegation (25)
        Length: 12
        Value: d0cc94090000000000000000
        IAID: d0cc9409
        T1: 0
        T2: 0
    Option Request
        Option: Option Request (6)
        Length: 10
        Value: 001700180038001f000e
        Requested Option code: DNS recursive name server (23)
        Requested Option code: Domain Search List (24)
        Requested Option code: NTP Server (56)
        Requested Option code: Simple Network Time Protocol Server (31)
        Requested Option code: Rapid Commit (14)
    Client Identifier
        Option: Client Identifier (1)
        Length: 14
        Value: 00020000ab11d258482fc7eee651
        DUID: 00020000ab11d258482fc7eee651
        DUID Type: assigned by vendor based on Enterprise number (2)
        Enterprise ID: Tom Gundersen (systemd) (43793)
        Identifier: d258482fc7eee651
    Elapsed time
        Option: Elapsed time (8)
        Length: 2
        Value: 0bd0
        Elapsed time: 30240ms