cmd/tailscale,ipn: add relay-server-port "tailscale set" flag and Prefs field (#15594)

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>

ipn/ipn_clone.go

@@ -61,6 +61,9 @@ func (src *Prefs) Clone() *Prefs {
 			}
 		}
 	}
+	if dst.RelayServerPort != nil {
+		dst.RelayServerPort = ptr.To(*src.RelayServerPort)
+	}
 	dst.Persist = src.Persist.Clone()
 	return dst
 }
@@ -96,6 +99,7 @@ var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	PostureChecking        bool
 	NetfilterKind          string
 	DriveShares            []*drive.Share
+	RelayServerPort        *int
 	AllowSingleHosts       marshalAsTrueInJSON
 	Persist                *persist.Persist
 }{})