net/interfaces: redo how we get the default interface on macOS and iOS

With #6566 we added an external mechanism for getting the default
interface, and used it on macOS and iOS (see tailscale/corp#8201).
The goal was to be able to get the default physical interface even when
using an exit node (in which case the routing table would say that the
Tailscale utun* interface is the default).

However, the external mechanism turns out to be unreliable in some
cases, e.g. when multiple cellular interfaces are present/toggled (I
have occasionally gotten my phone into a state where it reports the pdp_ip1
interface as the default, even though it can't actually route traffic).

It was observed that `ifconfig -v` on macOS reports an "effective interface"
for the Tailscale utn* interface, which seems promising. By examining
the ifconfig source code, it turns out that this is done via a
SIOCGIFDELEGATE ioctl syscall. Though this is a private API, it appears
to have been around for a long time (e.g. it's in the 10.13 xnu release
at https://opensource.apple.com/source/xnu/xnu-4570.41.2/bsd/net/if_types.h.auto.html)
and thus is unlikely to go away.

We can thus use this ioctl if the routing table says that a utun*
interface is the default, and go back to the simpler mechanism that
we had before #6566.

Updates #7184
Updates #7188

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

ipn/ipnlocal/local.go

@@ -3840,7 +3840,6 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 
 	// See the netns package for documentation on what this capability does.
 	netns.SetBindToInterfaceByRoute(hasCapability(nm, tailcfg.CapabilityBindToInterfaceByRoute))
-	interfaces.SetDisableAlternateDefaultRouteInterface(hasCapability(nm, tailcfg.CapabilityDebugDisableAlternateDefaultRouteInterface))
 	netns.SetDisableBindConnToInterface(hasCapability(nm, tailcfg.CapabilityDebugDisableBindConnToInterface))
 
 	b.setTCPPortsInterceptedFromNetmapAndPrefsLocked(b.pm.CurrentPrefs())