[macvlan] Same parent and gateway for multiple networks

### Description

I'm remaking this from https://github.com/moby/libnetwork/issues/2384

It's still relevant to me (esp with ipv6 networking) and I'd be happy to try working on it, but wanted to check first if the solution proposed there had any issues.