pimd: Add ability to set DR Priority for an interface

pimd: Add ability to set DR Priority for an interface

From RFC 4601, Section 4.3.1:

The DR_Priority Option allows a network administrator to give
preference to a particular router in the DR election process by
giving it a numerically larger DR Priority.  The DR_Priority Option
SHOULD be included in every Hello message, even if no DR Priority is
explicitly configured on that interface.  This is necessary because
priority-based DR election is only enabled when all neighbors on an
interface advertise that they are capable of using the DR_Priority
Option.  The default priority is 1.

This modification allows the setting of the DR Priority on a per
interface basis.

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