pimd: Stop DR election on every hello

pimd: Stop DR election on every hello

The DR election is occurring on every hello received.
This is because the hello receive packet returns a 0
for any successfully received packet.  PIMD then looked
at the 0 returned and did a DR election.

Code was inspected for the cases where DR should happen:
(A) Interface ip address change
(B) DR priority in hello packet changes
(C) Received a new neighbor on an interface
(D) Neighbor timer pops.

Each of these initiate a DR election in the code currently.

Testing was initiated on a pim network:
tor-11# show ip pim designated-router
NonPri: Number of neighbors missing DR Priority hello option

Interface Address         DR              Uptime   Elections Changes NonPri
br1       20.0.15.1       20.0.15.1       00:08:16         1       1      0
swp1      169.254.0.10    169.254.0.10    00:08:16         2       1      0
swp2      169.254.0.26    169.254.0.26    00:08:16         2       1      0
tor-11#

As you can see Elections == 2.  This is because pimd performs
an election on (A) and (C) above.  I see no need to modify
(A) to check if we have any knowledge of the interface before
this call.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>