util/syspolicy: finish plumbing policyclient, add feature/syspolicy, move global impl

This is step 4 of making syspolicy a build-time feature.

This adds a policyclient.Get() accessor to return the correct
implementation to use: either the real one, or the no-op one. (A third
type, a static one for testing, also exists, so in general a
policyclient.Client should be plumbed around and not always fetched
via policyclient.Get whenever possible, especially if tests need to use
alternate syspolicy)

Updates #16998
Updates #12614

Change-Id: Iaf19670744a596d5918acfa744f5db4564272978
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager.go

@@ -30,6 +30,7 @@ import (
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/slicesx"
+	"tailscale.com/util/syspolicy/policyclient"
 )
 
 var (
@@ -576,7 +577,7 @@ func (m *Manager) FlushCaches() error {
 //
 // health must not be nil
 func CleanUp(logf logger.Logf, netMon *netmon.Monitor, health *health.Tracker, interfaceName string) {
-	oscfg, err := NewOSConfigurator(logf, nil, nil, interfaceName)
+	oscfg, err := NewOSConfigurator(logf, health, policyclient.Get(), nil, interfaceName)
 	if err != nil {
 		logf("creating dns cleanup: %v", err)
 		return