bgpd: add bgp default link-state command

bgpd: add bgp default link-state command

Add the "bgp default link-state" command to the "router bgp" context.

> router bgp 65000
>  bgp default link-state

When this command is set, the "link-state/link-state" AFI/SAFI is
activated on all neighbors that are directly specified within the
"router bgp" unless explicitly deactivated:

> router bgp 65000
>  bgp default link-state
>  neighbor 10.0.0.1 remote-as 65001
>  address-family link-state link-state
>   no neighbor 10.0.0.1 activate

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>