This flag is currently no-op and hidden. The flag does round trip
through the related pref. Subsequent commits will tie them to
net/udprelay.Server. There is no corresponding "tailscale up" flag,
enabling/disabling of the relay server will only be supported via
"tailscale set".
This is a string flag in order to support disablement via empty string
as a port value of 0 means "enable the server and listen on a random
unused port". Disablement via empty string also follows existing flag
convention, e.g. advertise-routes.
Early internal discussions settled on "tailscale set --relay="<port>",
but the author felt this was too ambiguous around client vs server, and
may cause confusion in the future if we add related flags.
Updates tailscale/corp#27502
Signed-off-by: Jordan Whited <jordan@tailscale.com>
e17abbf461