pim-anycast-rp: Add limited support for secondary addresses.

pim-anycast-rp: Add limited support for secondary addresses.

Anycast requires that the lo interface be associated with multiple
addresses. One is the anycast IP address (which is the same on all RPs
participating in RP redundancy) and the second is the unique IP address
that will be used as the router id by routing protocols.

To accomodate that we maintain a list of secondary addresses per-pim iface
and allow any of them to be the RP address. This lets the I_am_RP macro
succeed on anycast RPs.

Note that the support is limited i.e. we don't actually advertise a
secondary list to the neighbors. This is assuming the anycast IP will never
be used as a router id i.e. will never be an RPF neighbor.

Sample output:
==============
dell-s6000-04# sh ip pim interface lo
Interface : lo
State     : up
Address   : 100.1.1.1 (primary)
            100.1.1.2
            100.1.1.3
            100.1.2.1
>>>>>>> SNIP >>>>>>>>>>>>>>>
dell-s6000-04# sh ip pim interface lo json
{
  "lo":{
    "name":"lo",
    "state":"up",
    "address":"100.1.1.1",
    "index":1,
    "lanDelayEnabled":true,
    "secondaryAddressList":[
      "100.1.1.2",
      "100.1.1.3",
      "100.1.2.1"
    ],
>>>>>>> SNIP >>>>>>>>>>>>>>>
dell-s6000-04#sh ip pim rp-info
RP address       group/prefix-list   OIF         I am RP
100.1.2.1        224.0.0.0/4         lo          yes
dell-s6000-04#

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>