device: add Device.ListenPort and Device.SetListenPort

device: add Device.ListenPort and Device.SetListenPort

This is a sample commit for a possible way to make
a Go API that lives alongside UAPI.

The general idea is to add Device and Peer methods
corresponding to UAPI directives, including a way to
look up a peer from a device based on a public key,
as in UAPI.

The UAPI code then deals with parsing and generating textual
input/output, and calls the Go methods to do the work.

This commit also contains a bug fix for a racy access of device.net.port
I will send an independently commit that fixes those directly in UAPI.
This commit is NOT meant to be merged as-is.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>