Adding a subcommand which prints and logs a log marker. This should help
diagnose any issues that users face.
Fixes#1466
Signed-off-by: Maisem Ali <maisem@tailscale.com>
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>