Instead of having the CLI check whether IP forwarding is enabled, ask
tailscaled. It has a better idea. If it's netstack, for instance, the
sysctl values don't matter. And it's possible that only the daemon has
permission to know.
Fixes#1626
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Without this, `tailscale status` ignores the --socket flag on macOS and
always talks to the IPNExtension, even if you wanted it to inspect a
userspace tailscaled.
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
