net/routetable: include unknown flags in the routetable doctor output

As part of the work on #7248 I wanted to know all of the flags on the
RouteMessage struct that we get back from macOS. Though it doesn't turn
out to be useful (when using an exit node/Tailscale is the default route,
the flags for the physical interface routes are the same), it still seems
useful from a debugging/comprehensiveness perspective.

Adds additional Darwin flags that were output once I enabled this mode.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/routetable/routetable_bsd.go

@@ -64,6 +64,16 @@ func (r RouteEntryBSD) Format(f fmt.State, verb rune) {
 		}
 		pr("Flags: %v", r.Flags)
 
+		unknownFlags := r.RawFlags
+		for fv := range flags {
+			if r.RawFlags&fv == fv {
+				unknownFlags &= ^fv
+			}
+		}
+		if unknownFlags != 0 {
+			pr("UnknownFlags: %x ", unknownFlags)
+		}
+
 		w.WriteString("}")
 	}).Format(f, verb)
 }