ip6-nd: set router flag on NA if appropriate

ip6-nd: set router flag on NA if appropriate

Type: fix

The router flag on a neighbor advertisement can be used by neighbors to
detect that a router has changed to a host (RFC 4861 section 4.4).

If a neighbor adds routes after receiving a router advertisement sent
by VPP and subsequently receives a neighbor advertisement sent by VPP,
it may remove any routes it added based on the RA if the NA does not
have the router flag set. It appears that this is how windows behaves.

When sending a neighbor advertisement, set the router flag if sending
RAs is enabled on the interface.

Signed-off-by: Matthew Smith <mgsmith@netgate.com>
Change-Id: I1f3e42bbd8ea1a4c116b1ce5a8273652d4cd763d