ospf6d: fix interface area configuration

ospf6d: fix interface area configuration

Currently the interface area is configured from the router node using
"interface IFNAME area ID" command. There are multiple problems with
this command:
- it is not in line with all other interface-related commands - other
  parameters are configured from the interface node using "ipv6 ospf6"
  prefix
- it is not in line with OSPFv2 - area is configured from the interface
  node using "ip ospf area" command
- most importantly, it doesn't work correctly when the interface is in
  a different VRF - instead of configuring the interface, it creates a
  new fake interface and configuring it instead

To fix all the problems, this commit adds a new command to the interface
configuration node - "ipv6 ospf6 area ID". The purpose of the command is
completely the same, but it works correctly in a multi-VRF environment.

The old command is preserved for the backward compatibility, but the
warning is added that it is deprecated because it doesn't work correctly
with VRFs.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>