logpolicy,tsnet: remove syspolicy dependency

tsnet depends on logpolicy, which in turn depended on util/syspolicy
because of a single LogTarget policy setting it uses.

In this commit, we replace that dependency with a feature.Hook,
which only tailscaled or its platform-specific alternatives should set.

Updates #20031

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -308,7 +308,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/feature/runtimemetrics                         from tailscale.com/feature/condregister
    L    tailscale.com/feature/sdnotify                               from tailscale.com/feature/condregister
   LD    tailscale.com/feature/ssh                                    from tailscale.com/cmd/tailscaled
-        tailscale.com/feature/syspolicy                              from tailscale.com/feature/condregister+
+        tailscale.com/feature/syspolicy                              from tailscale.com/feature/condregister
         tailscale.com/feature/taildrop                               from tailscale.com/feature/condregister
         tailscale.com/feature/tailnetlock                            from tailscale.com/feature/condregister
    L    tailscale.com/feature/tap                                    from tailscale.com/feature/condregister

cmd/tailscaled/tailscaled.go

@@ -438,6 +438,10 @@ func run() (err error) {
 
 	var publicLogID logid.PublicID
 	if buildfeatures.HasLogTail {
+		logpolicy.GetLogTarget.Set(func() string {
+			target, _ := sys.PolicyClientOrDefault().GetString(pkey.LogTarget, "")
+			return target
+		})
 
 		pol := logpolicy.Options{
 			Collection: logtail.CollectionNode,