Basically, don't trust the OS-level link monitor to only tell you
interesting things. Sanity check it.
Also, move the interfaces package into the net directory now that we
have it.
* adds new packet "netcheck" to do the checking of UDP, IPv6, and
nearest DERP server, and the Report type for all that (and more
in the future, probably pulling in danderson's natprobe)
* new tailcfg.NetInfo type
* cmd/tailscale netcheck subcommand (tentative name, likely to
change/move) to print out the netcheck.Report.
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick
michael wilson