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_openbsd.go

@@ -11,6 +11,7 @@ import (
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/syspolicy/policyclient"
 )
 
 type kv struct {
@@ -24,7 +25,7 @@ func (kv kv) String() string {
 // NewOSConfigurator created a new OS configurator.
 //
 // The health tracker may be nil; the knobs may be nil and are ignored on this platform.
-func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
+func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ policyclient.Client, _ *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
 	return newOSConfigurator(logf, health, interfaceName,
 		newOSConfigEnv{
 			rcIsResolvd: rcIsResolvd,