Full ietf-interfaces YANG support (advanced)

[troglobit]

Tasks

• [ ] ietf-ip: ip:neighbor
• [x] ietf-ip: ipv4+ipv6 mtu support
• [ ] ietf-intefaces: last-change
• [ ] ietf-intefaces: higher-layer-if
• [ ] ietf-intefaces: lower-layer-if
• [x] #152
• [ ] #515