router_linux: use only baseline 'ip rule' features that exist in old kernels.

This removes the use of suppress_ifgroup and fwmark "x/y" notation,
which are, among other things, not available in busybox and centos6.

We also use the return codes from the 'ip' program instead of trying to
parse its output.

I also had to remove the previous hack that routed all of 100.64.0.0/10
by default, because that would add the /10 route into the 'main' route
table instead of the new table 88, which is no good. It was a terrible
hack anyway; if we wanted to capture that route, we should have
captured it explicitly as a subnet route, not as part of the addr. Note
however that this change affects all platforms, so hopefully there
won't be any surprises elsewhere.

Fixes #405
Updates #320, #144

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

ipn/local.go

@@ -777,12 +777,8 @@ func routerConfig(cfg *wgcfg.Config, prefs *Prefs, dnsDomains []string) *router.
 	var addrs []wgcfg.CIDR
 	for _, addr := range cfg.Addresses {
 		addrs = append(addrs, wgcfg.CIDR{
-			IP: addr.IP,
-			// TODO(apenwarr): this shouldn't be hardcoded in the client
-			// TODO(danderson): fairly sure we can make this a /32 or
-			// /128 based on address family. Need to check behavior on
-			// !linux OSes.
-			Mask: 10,
+			IP:   addr.IP,
+			Mask: 32,
 		})
 	}