zebra: add IPv6 router-id

zebra: add IPv6 router-id

 * add a vrf sub-command `[no] ipv6 router-id X:X::X:X`.
 * add command `[no] ipv6 router-id X:X::X:X [vrf NAME]` for backward
   compatibility.
 * add a vrf sub-command `[no] ip router-id A.B.C.D` and make the old
   one without `ip` an alias for it.
 * add a  command `[no] ip router-id A.B.C.D [vrf NAME]` for backward
   comptibility and make the old one without `ip` an alias for it.
 * add command `show ip router-id [vrf NAME]` and make
   the old one without `ip` an alias for it.
 * add command `show ipv6 router-id [vrf NAME]`.
 * add ZAPI commands `ZEBRA_ROUTER_ID_V6_ADD`,
   `ZEBRA_ROUTER_ID_V6_DELETE` and `ZEBRA_ROUTER_ID_V6_UPDATE`
   for deamons to get notified of the IPv6 router-id.
 * update zebra documentation.

Signed-off-by: Sebastien Merle <sebastien@netdef.org>