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>

tsnet/tsnet_test.go

@@ -2926,6 +2926,7 @@ func TestDeps(t *testing.T) {
 		BadDeps: map[string]string{
 			"golang.org/x/crypto/ssh":                       "tsnet should not depend on SSH",
 			"golang.org/x/crypto/ssh/internal/bcrypt_pbkdf": "tsnet should not depend on SSH",
+			"tailscale.com/feature/syspolicy":               "tsnet should not depend on syspolicy",
 			"tailscale.com/ipn/store/awsstore":              "tsnet callers wanting AWS state storage should import awsstore themselves",
 			"tailscale.com/ipn/store/kubestore":             "tsnet callers wanting Kubernetes state storage should import kubestore themselves",
 			"tailscale.com/wif":                             "tsnet callers wanting workload identity federation should import tailscale.com/feature/identityfederation themselves",