Until we turn on AAAA by default (which might make some people rely on
Happy Eyeballs for targets without IPv6), this lets people turn it on
explicitly if they want.
We still should add a peer cap as well in the future to let a peer
explicitly say that it's cool with IPv6.
Related: #9574
Updates #1813
Updates #1152
Change-Id: Iec6ec9b4b5db7a4dc700ecdf4a11146cc5303989
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
597d0e8fd5