dhcpv6: pretty message printing

[hugelgupf]

cleaned up message printing in DHCPv6. I chose some I think somewhat readable format. I kept the {} because messages can nest so deep, imho they're really useful.

I didn't expand IANA and IAPD fields on their own line -- let me know what you think about that...

examples/everythingmessage:

RelayMessage{                          
  MessageType=RELAY-FORW   
  HopCount=0             
  LinkAddr=<nil>  
  PeerAddr=<nil>                       
  Options: [                                                                                                                                                            
    Relay Message: Message{
      MessageType=REPLY                                                             
      TransactionID=0x6c95c4
      Options: [                                                                    
        Client ID: DUID{Type=DUID-LLT HWType=Ethernet HWAddr=01:02:de:ad:be:ef}                                                                                         
        Rapid Commit                                                                
        Boot File URL: http://foobar                                                
        Boot File Parameters: [loglevel=10 uroot.nohwrng]
        Client Link-Layer Address: {Type=Ethernet LinkLayerAddress=01:02:be:ef:de:ad}
        4RD: {Options=[                   
          4RD Mapping Rule: {Prefix4=123.123.0.0/16, Prefix6=fc80::/64, EA-Bits=0, WKPAuthorized=false}                                                                
          4RD Non-Mapping Rule: {HubAndSpoke=true, TrafficClass=false, DomainPMTU=0}                                                                                    
        ]}                                                                                                                                                              
        Encapsulated DHCPv4 Message: {DHCPv4 Message                                
          opcode: BootRequest
          hwtype: Ethernet
          hopcount: 0
          transaction ID: 0xc4077d78                                                
          num seconds: 0                                                            
          flags: Unicast (0x00)
          client IP: 0.0.0.0                                                        
          your IP: 0.0.0.0
          server IP: 0.0.0.0
          gateway IP: 0.0.0.0
          client MAC: 01:02:de:ad:be:ef
          server hostname: 
          bootfile name: 
          options:
            DHCP Message Type: DISCOVER
            Parameter Request List: Subnet Mask, Router, Domain Name Server, Domain Name                                                                                
        }                  
        DHCPv4-over-DHCPv6 Servers: [fe81::1]                                       
        DNS: [fe82::1]      
        IAPD: {IAID=0x01020304 T1=0s T2=0s Options=[                                
          IAPrefix: {PreferredLifetime=0s ValidLifetime=0s Prefix=fc80::/64 Options=[                                                                                   
            StatusCode: {Code=Success (0); Message=yeah whatever}                   
          ]}                                                                        
        ]}                                                                          
        Information Refresh Time: 1s                                                
        Interface ID: [1 2]
        Network Interface ID: First gen. PXE boot ROMs (Revision 1.0)               
        Vendor Options: {EnterpriseNum=123 VendorOptions=[                          
          unknown (400): [102 111 111 98 97 114]                                
        ]}           
        IANA: {IAID=0xdeadbeef T1=0s T2=0s Options=[
          IAAddress: {IP=fe80::1 PreferredLifetime=0s ValidLifetime=0s Options=[
            StatusCode: {Code=Success (0); Message=yes}
          ]}
        ]}
        IATA: {IAID=0x00000000 Options=[
          IAAddress: {IP=fe80::1 PreferredLifetime=0s ValidLifetime=0s Options=[
            StatusCode: {Code=Success (0); Message=yes}
          ]}
        ]}
        IATA: {IAID=0x00000000 Options=[]}                                                               
      ]     
    }     
    Remote ID: {EnterpriseNumber=291 RemoteID=[1 2]}                                
  ]                                                                                 
}                    

[pmazzini]

stickler-ci is not happy. There are some LongString methods without a comment.

[hugelgupf]

Closing in favor of #485