util/syspolicy/*: move syspolicy keys to new const leaf "pkey" package

This is step 1 of ~3, breaking up #14720 into reviewable chunks, with
the aim to make syspolicy be a build-time configurable feature.

In this first (very noisy) step, all the syspolicy string key
constants move to a new constant-only (code-free) package. This will
make future steps more reviewable, without this movement noise.

There are no code or behavior changes here.

The future steps of this series can be seen in #14720: removing global
funcs from syspolicy resolution and using an interface that's plumbed
around instead. Then adding build tags.

Updates #12614

Change-Id: If73bf2c28b9c9b1a408fe868b0b6a25b03eeabd1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logpolicy/logpolicy.go

@@ -52,6 +52,7 @@ import (
 	"tailscale.com/util/must"
 	"tailscale.com/util/racebuild"
 	"tailscale.com/util/syspolicy"
+	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/testenv"
 	"tailscale.com/version"
 	"tailscale.com/version/distro"
@@ -65,7 +66,7 @@ var getLogTargetOnce struct {
 func getLogTarget() string {
 	getLogTargetOnce.Do(func() {
 		envTarget, _ := os.LookupEnv("TS_LOG_TARGET")
-		getLogTargetOnce.v, _ = syspolicy.GetString(syspolicy.LogTarget, envTarget)
+		getLogTargetOnce.v, _ = syspolicy.GetString(pkey.LogTarget, envTarget)
 	})
 
 	return getLogTargetOnce.v