bgpd: Implement graceful-shutdown command per neighbor

bgpd: Implement graceful-shutdown command per neighbor

We already have a global knob for graceful-shutdown, but it's handy having
per neighbor knob as well.

Especially when a single neighbor needs to be restarted/shutdown gracefuly.

We can do this route-maps, but this is a faster/cleaner way doing the same
for an operator.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>