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/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,