ospfd: preserve DR status across graceful restarts

ospfd: preserve DR status across graceful restarts

RFC 3623 says:
 "If the restarting router determines that it was the Designated
 Router on a given segment prior to the restart, it elects
 itself as the Designated Router again.  The restarting router
 knows that it was the Designated Router if, while the
 associated interface is in Waiting state, a Hello packet is
 received from a neighbor listing the router as the Designated
 Router".

Implement that logic when processing Hello messages to ensure DR
interfaces will preserve their DR status across a graceful restart.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>