Not great, but lets people working on new ports get going more quickly
without having to do everything up front.
As the link monitor is getting used more, I felt bad having a useless
implementation.
Updates #815
Updates #1427
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
And make the monitor package portable with no-op implementations on
unsupported operating systems.
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
