Limit router gw ports' stateful fixed IPs to one per address family

Limit router gw ports' stateful fixed IPs to one per address family

Validate a router's gateway port during a router update by ensuring
it has no more than one v4 fixed IP and one v6 (statefully-assigned)
fixed IP.

Note that there is no limit on v6 addresses from SLAAC and
DHCPv6-stateless subnets as they are automatically allocated.

Change-Id: I6a328048b99af39ab9497fd9f265d1a9b95b7148
Closes-Bug: 1438819
Partially-implements: blueprint multiple-ipv6-prefixes
(cherry picked from commit 1bfd86e1ef7148370798aa99c868d7f931fcbf78)