This PR plumbs through awareness of an IPv6 SNAT/masquerade address from the wire protocol
through to the low-level (tstun / wgengine). This PR is the first in two PRs for implementing
IPv6 NAT support to/from peers.
A subsequent PR will implement the data-plane changes to implement IPv6 NAT - this is just plumbing.
Signed-off-by: Tom DNetto <tom@tailscale.com>
Updates ENG-991