BGP: Trigger IPv6 router advertisements upon config of unnumbered neighbor

BGP: Trigger IPv6 router advertisements upon config of unnumbered neighbor

Instead of turning on IPv6 RA on every interface as soon as it has an IPv6
address, only enable it upon configuration of BGP neighbor. When the BGP
neighbor is deleted, signal that RAs can be turned off.

To support this, introduce new message interaction between BGP and Zebra.
Also, take appropriate actions in BGP upon interface add/del since the
unnumbered neighbor could exist prior to interface creation etc.

Only unnumbered IPv6 neighbors require RA, the /30 or /31 based neighbors
don't. However, to keep the interaction simple and not have to deal with
too many dynamic conditions (e.g., address deletes or neighbor change to/from
'v6only'), RAs on the interface are triggered upon any unnumbered neighbor
configuration.

BGP-triggered RAs will cause RAs to be initiated on the interface; however,
if BGP asks that RAs be stopped (upon delete of unnumbered neighbor), RAs
will continue to be exchanged if the operator has explicitly enabled.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-10640
Reviewed By: CCR-4589
Testing Done: Various manual and automated (refer to defect)