net/interfaces: make syscall and netstat agree when multiple gateways are present

likelyHomeRouterIPDarwinSyscall iterates through the list of routes,
looking for a private gateway, returning the first one it finds.

likelyHomeRouterIPDarwinExec does the same thing,
except that it returns the last one it finds.

As a result, when there are multiple gateways,
TestLikelyHomeRouterIPSyscallExec fails.
(At least, I think that that is what is happening;
I am going inferring from observed behavior.)

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

net/interfaces/interfaces_darwin.go

@@ -5,6 +5,7 @@
 package interfaces
 
 import (
+	"errors"
 	"os/exec"
 
 	"go4.org/mem"
@@ -62,8 +63,12 @@ func likelyHomeRouterIPDarwinExec() (ret netaddr.IP, ok bool) {
 		ip, err := netaddr.ParseIP(string(mem.Append(nil, ipm)))
 		if err == nil && isPrivateIP(ip) {
 			ret = ip
+			// We've found what we're looking for.
+			return stopReadingNetstatTable
 		}
 		return nil
 	})
 	return ret, !ret.IsZero()
 }
+
+var stopReadingNetstatTable = errors.New("found private gateway")