Add support for HSRPv1 Advertise packets

[amlamarra]

Scapy was not properly recognizing HSRPv1 Advertisement (opcode 3) packets. The fields are different in these packets.

It doesn't help that the Advertise packets are not detailed in the RFC (2281).

This MR adds support for such packets.

Before:

>>> pkts = rdpcap("hsrp.pcap")
>>> pkts.show()
0000 Ether / IP / UDP 10.28.165.252:1985 > 224.0.0.2:1985 / HSRP / Padding
0001 Ether / IP / UDP 10.28.169.252:1985 > 224.0.0.2:1985 / Raw / Padding

After:

>>> pkts = rdpcap("hsrp.pcap")
>>> pkts.show()
0000 Ether / IP / UDP 10.28.165.252:1985 > 224.0.0.2:1985 / HSRP / Padding
0001 Ether / IP / UDP 10.28.169.252:1985 > 224.0.0.2:1985 / HSRP / Padding
>>> pkts[1].show()
###[ Ethernet ]###
  dst       = 01:00:5e:00:00:02
  src       = 00:12:7f:ba:1f:02
  type      = IPv4
###[ IP ]###
     version   = 4
     ihl       = 5
     tos       = 0xc0
     len       = 44
     id        = 0
     flags     =
     frag      = 0
     ttl       = 1
     proto     = udp
     chksum    = 0x24e7
     src       = 10.28.169.252
     dst       = 224.0.0.2
     \options   \
###[ UDP ]###
        sport     = 1985
        dport     = 1985
        len       = 24
        chksum    = 0xeb0d
###[ HSRP ]###
           version   = 0
           opcode    = Advertise
           adv_type  = HSRP interface state
           adv_length= 14
           adv_state = Passive
           adv_reserved1= 0
           adv_active_grps= 0
           adv_passive_grps= 1
           adv_reserved2 = 1862270976
###[ Padding ]###
              load      = b'\x00\x00'

[p-l-]

Rather than having many ConditionalFields, maybe you may try having two distinct layers (say, HSRP and HSRPAdvertise) and use a dispatch_hook() method?

[amlamarra]

Thanks, I had never used dispatch_hook() before.