net/netmon, wgengine/userspace: purge ChangeDelta.Major and address TODOs (#17823)

updates tailscale/corp#33891

Addresses several older the TODO's in netmon.  This removes the 
Major flag precomputes the ChangeDelta state, rather than making
consumers of ChangeDeltas sort that out themselves.   We're also seeing
a lot of ChangeDelta's being flagged as "Major" when they are
not interesting, triggering rebinds in wgengine that are not needed.  This
cleans that up and adds a host of additional tests.

The dependencies are cleaned, notably removing dependency on netmon
itself for calculating what is interesting, and what is not.  This includes letting
individual platforms set a bespoke global "IsInterestingInterface"
function.  This is only used on Darwin.

RebindRequired now roughly follows how "Major" was historically
calculated but includes some additional checks for various
uninteresting events such as changes in interface addresses that
shouldn't trigger a rebind.  This significantly reduces thrashing (by
roughly half on Darwin clients which switching between nics).   The individual
values that we roll  into RebindRequired are also exposed so that
components consuming netmap.ChangeDelta can ask more
targeted questions.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

net/netmon/state.go

@@ -149,12 +149,28 @@ type Interface struct {
 	Desc     string     // extra description (used on Windows)
 }
 
-func (i Interface) IsLoopback() bool { return isLoopback(i.Interface) }
-func (i Interface) IsUp() bool       { return isUp(i.Interface) }
+func (i Interface) IsLoopback() bool {
+	if i.Interface == nil {
+		return false
+	}
+	return isLoopback(i.Interface)
+}
+
+func (i Interface) IsUp() bool {
+	if i.Interface == nil {
+		return false
+	}
+	return isUp(i.Interface)
+}
+
 func (i Interface) Addrs() ([]net.Addr, error) {
 	if i.AltAddrs != nil {
 		return i.AltAddrs, nil
 	}
+	if i.Interface == nil {
+		return nil, nil
+	}
+
 	return i.Interface.Addrs()
 }
 
@@ -271,6 +287,9 @@ type State struct {
 
 	// PAC is the URL to the Proxy Autoconfig URL, if applicable.
 	PAC string
+
+	// TailscaleInterfaceIndex is the index of the Tailscale interface
+	TailscaleInterfaceIndex int
 }
 
 func (s *State) String() string {
@@ -485,6 +504,16 @@ func getState(optTSInterfaceName string) (*State, error) {
 		ifUp := ni.IsUp()
 		s.Interface[ni.Name] = ni
 		s.InterfaceIPs[ni.Name] = append(s.InterfaceIPs[ni.Name], pfxs...)
+
+		// Skip uninteresting interfaces.
+		if IsInterestingInterface != nil && !IsInterestingInterface(ni, pfxs) {
+			return
+		}
+
+		if isTailscaleInterface(ni.Name, pfxs) {
+			s.TailscaleInterfaceIndex = ni.Index
+		}
+
 		if !ifUp || isTSInterfaceName || isTailscaleInterface(ni.Name, pfxs) {
 			return
 		}